Saw some stupid "not an advertisement advertisement" article thing online the other day from Mens Health. Dude basically said to do pull-ups with your legs straight and feet slightly forward instead of bending at the knees with your feet behind you. I've never really even thought about it much and have always bent my knees and just tried to be as smooth as possible. Anywho, it didn't seem to make me do any less or more, but I really noticed it in my core/abs the next day. I was a little sore which made sense as it was a bit harder keeping my feet forward instead of behind. I'm sure i'll see something that says they should be behind you for some dumb reason, but i'll prob start doing them straight legged for a while.