Funny, I thought about posting something similar for quite a while:
Shaf made this post reporting results from a study which shows that the socalled "Janda situp" does not inhibit the psoas, I have no time now but will post a summary of what Janda wrote about this exercise latter. Just short: Janda AFAIK NEVER claimed so, it was more about the correct timing of abs and psoas and supporting the legs from the back was just a help if you can't do it from the get go. The aim was to do it without any support on the legs. Latter more.