A blue gear of oval shape, rotating around fixed axis, engages with yellow gear of a gear–pulley block. The latter has revolution joint with the green angle arm that can rock around a fixed axis.
Orange slider can reciprocate in the slot of the green angle arm.
Input is pink pulley. The blue oval gear rotates irregularly. Brown bar reciprocates with dwell.
Weight of the brown bar (or spring) maintains permanent engagement of the gear drive.
Input can be the blue oval gear. In that case the belt drive isn’t needed.