Source: Peter Kostis
In your backswing especially. Once you get your flex at address, you want to feel like your right knee cap doesn't move much as you take the club back. If you really let it straighten, and let this leg lock, sometimes all you're gonna do is make this over the top move. So try to maintain some flex in your knees as you go back.
Source: Nick Faldo
The right knee has to be solid
*Nick hits a drive*
Arhh... a little bit too much fade
No wonder he swung out-to-in if his hips didn't turn properly, rotating too early on the downswing and pulling the arms across the ball.
Here is a montage of the 20 best players in the world (except Furyk, his swing is too weird). With the exception of 3-4 players, everyone straighten the right leg pretty much. Not so far that they lock it, which you should not do, but definitely not keeping the same flex as at address.