Hello...skypig.
The upper back should be put into rigid extension and held in that position for each rep. We can cue this by telling the lifter to lift their chest and getting the thoracic erectors to contract and hold. Some people can simply hold it rigid with little effort and some people need to be reminded to reset it every rep. The problem that can occur is if lifting the chest to set the upper back starts to put the lumbar spine in over extension. That causes other issues that need to be dealt with.
Post a video, using the sticky guidelines, and we can take a look.