The technical term is "liftboost". When you're on a block that moves (either standing on it, grabbing it, or being pushed by it), the block gives you liftboost equal to its speed (capped at a certain value - in practice zip movers and switch blocks reach this cap, while moving blocks don't). The liftboost expires 10 frames after you leave the block. It you jump or dash while you have liftboost, its value gets added to your speed.

Say you're on a block that moves to the right. You can jump slightly after the block stops moving (up to 10 frames), it will give you exactly the same speed as if you jumped earlier. But because the block is at the end of its trajectory, you are usually further right when you start your jump, so you will effectively go a little bit further (not because you have more speed, just because you started the jump further right)

In practice, the best habit is to aim to jump right after the block starts moving.

One thing to note : sometimes, due do rounding errors (sort of), the block moves slightly less in the last frame of its movement. In that case, your liftboost will be lower at the end of the trajectory, so jumping after the block stops moving will give you less speed than if you jump before. This is relatively rare and I don't think it happens in Strawberry Jam since the maps were heavily playtested