According to this video https://www.youtube.com/watch?v=uJje3yWcTdU when the bottom 4 bar lifts up the bottom gear will passively spin. How does this work exactly?
That bottom sprocket on the right hand side is fastened to the lower arm of the DR4B (Double Reverse 4-Bar.) This means that when the lift raises, it will rotate with the piece of metal it’s attached to. In that video, the sprocket is likely being used as a means to drive the lift by chain. The gears on the left hand side only exist to “link” the upper and lower sets of arms on the DR4B, ensuring that they move together and (at the end of the linkages) parallel to each other. The term “passively” as it describes the gears’ motion is kinda misleading, as they are driven, just not directly.
Hope that answers your question - if not, I’ll be happy to clarify.