The pivot pin is eliptical in shape, like a cam lobe, it will only lift out if the lobe is lined up with theopening, which only happens if the backrest is flat, which is why seat base removal makes it easier
To remove the base, lift up the front until it pops off the spring bars, then push back with your knees to disengage the spring bars from the rear hooks, at the same time, lift from the back, if that makes sense?
Courtesy of T-Cut
https://www.the75andztclub.co.uk/for...98&postcount=2