Why are most jigsaw blades designed to cut on the up stroke with teeth that point towards the shank? Why are reverse teeth jigsaw blades less popular? 
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1$\begingroup$ There's an underlying deeper question here of why we have straight sawblades that cut on the push stroke. Consider reading about the recent interest in Japanese saws like Ryouba. en.wikipedia.org/wiki/Japanese_saw $\endgroup$– CriggieAug 23, 2020 at 0:54
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$\begingroup$ I wonder how thick the pictured blade is, compared to a regular blade. $\endgroup$– CriggieAug 23, 2020 at 23:50
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2$\begingroup$ I understood up and down stroke as intended BUT based on various comments it may be useful to either qualify how the blade moves relative to the work and or refer to push and pull strokes relative to the saw body. | Push stroke places the blade in compression as the tip is pushed away from the saw body. Pull stroke places the blade in tension as the blade tip is pulled towards the saw body. $\endgroup$– Russell McMahonAug 24, 2020 at 5:13
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$\begingroup$ I'm surprised the answers below don't also mention the force on the user. Cutting on the up-stroke pulls the machine into the material, requiring minimal pressure from the user to keep it in place. Cutting on the down-stroke would push the machine away from the material, requiring significant downward force from the user to keep it in place. $\endgroup$– welfAug 26, 2020 at 6:40
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4 Answers
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The photo you have is not for the standard jigsaw blade. Standard blades teeth point upward and cut upward for the obvious reasons: less wobble, more control, center of force near the handle.
This is a revers cut blade designed to cut on downstroke to cut laminated work like countertops, so that the cut is not going to splinter the Formica or other laminate cover.
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$\begingroup$ Good point, but when I cut laminate I deeply score the cut line with a sharp Stanley knife (with a new blade) which avoids chipping of the final edge. $\endgroup$ Aug 22, 2020 at 5:49
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If you think what happens as each tooth cuts material then the blade is put into tension as it cuts, this means that the blade is likely to stay straight, but any unevenness between the teeth side to side may cause drift.
If the blade cuts on the downstroke it will buckle as it is relatively thin for its length. A blade designed to cut on the downstroke would have to either be thicker or the blade supported at both ends. This would mean removing (wasting) more material in each cut or limit the use of the machine as access would be needed to both sides of the material.
The blade cutting on the upstroke and the current design means that the cut can be achieved from one side of the material only.
My box of blades for metal and wood in different grades all have the teeth cutting in the up or pull direction and the roller support for the back of the blade defines “forward”.
Edit: Based on the comments I have included images of my jigsaw, one to show the shoe or baseplate and the roller for the back edge of the blade. The blade is almost in the fully down position and will cut on its next upstroke - note the direction of the teeth (that is a wood cutting blade fitted).
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1$\begingroup$ Why would the blade buckle if it's cutting on the downstroke but not the upstroke? That seems like the opposite of what should happen. If you've got something like a metal ruler, for instance, it'll bend if you put it into compression (pushing on it, like the upstroke) but remain straight if you put it into tension (pulling on it, like the downstroke). $\endgroup$ Aug 23, 2020 at 6:14
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7$\begingroup$ Try pushing a piece of cotton into a small hole and keep it straight, then a piece of stiff wire... And I think you may have compression and tension swapped in your comment. As per your first sentence the blade is cutting on the upstroke and will be in tension. $\endgroup$ Aug 23, 2020 at 6:54
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– hazzey ♦Aug 24, 2020 at 13:31
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1$\begingroup$ @nick012000 I think you have your logic backwards or you are imagining jigsaw configuration backwards. The upstroke pulls and the downstroke pushes. So the downstroke puts it in compression and the upstroke puts it in tension. Otherwise your observation of compression and tension is the exact reason jigsaw blades cut on the upstroke $\endgroup$ Aug 25, 2020 at 13:00
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To enlarge slightly upon Solar Mike's response, a jigsaw that cuts on the upstroke tends to yank the saw shoe down into firm contact with the workpiece. This guarantees that the cut angle will match the shoe angle; for a shoe angle of 90 degrees this means a nice square cut.
answered Aug 22, 2020 at 16:11
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5$\begingroup$ Contact with the shoe also stops vibration of the material which is very common with thin sheet. $\endgroup$ Aug 22, 2020 at 18:47
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To expand on Solar Mike's answer and to help explain part of niels nielsen's answer, the saw blade is normally designed to essentially squeeze the material between the teeth and the shoe. This generally makes for a more controlled cut. Circular saws, saws-alls, and most other mechanical saws work on this principle, too.
If you were to use the blade you have instead, you're looking at more "jumps", where the blade catches the material without cutting. This can happen with a normal jigsaw blade, but generally only if you try to force-feed the saw, turn too sharply, the material delaminates, the blade gets pinched, or a small handful of other reasons.
Because of the "squeeze", you don't have to put downward pressure on the saw to keep it cutting and avoid jumping. With your downward facing teeth, you'll have to go slower with cutting to avoid jumping or put force on the tool to keep it in contact with the material. Going slower is generally more acceptable, unless the material is just super hard and you have to force the blade into it to cut. But laminate, like Adam talks about, should be quite soft.
The other part of this where any tear-out is going to happen. With a normal blade, tear-out/splinters/ragged edges are going to happen on the top of the material, facing you. With downward facing teeth, the tear-out is going to be on the bottom of the material. Where you need/want that tear-out to happen depends on your project. If you are making a normal cut, you can simply flip the material over and cut from a drawing on the "bottom" of your material, leaving the good surface free from line marks, scratch marks from the saw shoe, tear-out, and most other marks that can come from building a project.
However, with laminates or veneers, like a kitchen counter or piece of furniture, you are going to be cutting from the top of the material, not the bottom, so you would want the downward facing blades to avoid chipping the top surface. Personally, I'm not sure I'd want to use a jigsaw for this purpose, but it might be the best tool out of a lot of bad options, too.
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$\begingroup$ Yup - try using a jigsaw when the foot plate is not pressed into the work. Its an unpleasant, high vibration experience, prone to dropping the blade out of the work completely and then immediately dinging the work with the end of the blade as it oscillates in free air. $\endgroup$– CriggieAug 25, 2020 at 8:53