[Weightshift]

Weightshift, I agree. However it is the left arm checkrein which prevent straightening before "both arms straight".

It is also because the left arm moves away from the left side of the body as you approach "both arms straight" which extends the right arm hence dissolving the checkrein.

So what I'm wondering, is if you take the left arm off the grip, then is there value in rehaearsing the stroke with a straight right arm - rehaearsing it with a bent/bending right arm is not representative of what your right arm should be doing in the real stroke (striving to remain straight) - although it may give the geometrical 'look' of what is happening.
Chris

Chris, personally I do not see any merit in this exercise, but others may disagree. A good exercise in my opinion would be to practise the backstroke and consequent downstroke to "both arms straight" using an impact bag. Hope this helps.