I don't want to sound too harsh, but do you even bother to read the safety card or watch the demo? You only put your head between your knees (or as far down as you can go if you've got a big belly in the way) if there is no seat in front of you. If there is a seat in front of you, you lean against it with your hands behind your head (fingers not interlocked).
See example in this
demo video from Virgin, at 2:40.
A normal forward facing passenger seatbelt only secures your lap, not your upper body/head. Therefore, if the forward motion of the plane stops suddenly (such as when skidding into a ditch/terminal building/catering truck), being already against the seat in front will stop your head slamming into it at full speed.
All those tests and investigations the aviation industry has done over the years shows it's a good idea to know the correct brace position (note it may vary given the seat/cabin design), and to adopt it when required.
Reventor: It is taken seriously by aviation professionals, but not by many passengers. Most pax think that 'oh, if the plane crashes I'm a gonna anyway', but actually if you pay a little attention you can survive many situations (even a ditching as we've all seen this week!).
Let's not debate in this thread why passengers don't have harnesses... why don't we have helmets, why don't we have smoke hoods and fire suits, why don't we have air bags, why don't we have parachutes... It's hard enough getting pax to do up their lap belt, I can't imagine how long it would take to secure a cabin if we waited for everyone to fasten a harness.