This technique allows you to do whatever you want with the girl, leaving your bro thinking you're going to wait for permission. When your bro says he didn't give you any, you can argue that you never actually asked for it anyway. Therefore a, "no, please don't fuck her, I really like this one", will not hold up in any argument. But if your bro says it's OK, you can pretend that you were in fact asking him if you could. You've gotta love win-win situations.