I believe what you are seeing is a pendulum mounted on a sliding bracket. The weight with the thumbscrew indicates that it can be tuned to match the required movement to engage the V-shaped "socket" into which it engages on alternating strokes.
Slowed down to one-quarter speed, the video shows that on one stroke, the pendulum is swinging left, while the sliding mechanism is pushing right, with inertia causing the engaging lever to rise. In the next half-stroke, the sliding mechanism pushes left with the lever in the socket, preventing inertial movement of the lever.
The next half-stroke to the right disengages the lever from the socket, gravity causing the lever to fall simultaneously with the pendulum causing the lever to fall with sufficient momentum to prevent the next left sliding stroke from engaging the socket.
I searched for various terms attempting to locate a specific term with no success.
I believe one can reproduce this mechanism by experimenting with varying angles between the lever and the pendulum arm and by varying distance on the pendulum weight.
I'd love to know what purpose the alternating actuation serves. One thinks it could be a two stroke engine system of sorts.