Isochoric means constant volume (In SI engine: piston should not move when the spark is ignited). In reality piston always keeps displacing up and down means volume changes. So, how does this make it a Isochoric Heat Addition in power stroke.
the rate at which heat is added is so fast that the piston has no opportunity to move during the process. Furthermore, when the piston is at the top of its stroke (which is where the heat is added) the crank-and-connecting rod mechanism yields a finite residence time of the piston in that position for a span of crank rotation, which means that even though the crank never stops rotating, the piston spends a finite amount of time in an almost motionless state at TDC.