Originally Posted by Bob Pataki
They don't leave the jaw exposed too much though, so taking a shot to the chin wont create that same jolt. In terms of the force it's pretty much the same, but I think it makes sense that it helps to prevent the jaw being clipped and rattling the head and neck.
The only thing it will really absorb well are straight punches that land at arm length, such as jabs. Those punches tend to be more "Impact" based than inertia based as the punchers arm will retract most of the energy once it reaches the end of the punch so there won't be much left to be transferred to the head of the opponent.
A big hook, uppercut or overhand, on the other hand, will have very, very little of it absorbed because of the mechanics of the punch. When a hook lands, the hand follows through the head with the rotation of the punchers body, transferring energy into the opponents head until the opponents head gains enough energy to travel with the punch (I.E. someone gets punched in the jaw and their face turns or they get punched in the temple and their head rotates sideways), that's the concussive force that is most likely to cause a knockout and head gear does next to nothing to prevent that.