Pedaling or coasting? Think back because the difference is critical. When you coast the cranks hold the front end of the chain, and the hub would spool the other to the upper loop, except that a working freehub prevents that. So the chain is stationary and there's no transfer of tension.

Pedaling is very different. Driving force keeps the upper loop tight, and the crank is spooling chain to the lower. If everything is right it runs through the RD and returns to the upper loop, but if, for example, there's a stiff link it can bind in the RD cage, causing slack in the lower loop. It pops back when the RD cage runs out of travel and the chain is pulled through by the cassette, causing a quick rebound.

This is a serious problem, because if the link is stiff enough you can pull the RD around the cassette to the top. It's easy to fix by lubing the chain.