This one was confusing me as well, thanks for asking for me UK Yank. So, this is what confuses me still:
Proper bike fit has the saddle set such that when your pedal is parallel with the seat tube, your leg should be just slightly bent. That means you wouldn’t have any room to stand up and still pedal fully. I get back on downhills, stay planted on climbs, and get off the seat for bumpy riding, but mostly right now I can’t pedal well when I am standing on the pedals. What am I doing incorrectly?