In this 2nd brace position, should your head lean against the seat in front?
The head should be against the seat in front of you. The reason is that if the plane crashes, your head and upper body will be thrown forwards until they hit something. If you're in a normal sitting position, your head will be moving pretty quickly by the time it smashes into the seat in front of you, leading to serious injury or death. Moreover, as your torso and head are moving forwards in an uncontrolled way, you can get injured by your head essentially flopping around. Since your head is going to end up against the seat in front of you anyway, putting it against that seat before the crash means it doesn't smash into it during the crash, and that it's not flopping around on its way to the seat.