If some one has you in a head lock please dont hit them in the groin unless you would like to die.. if some one is hit in the groin they tense up, if they are chocking you (with a head lock) them tensing up is gonna kill you.
Its sorta hard to explain with out a demonstration but the correct way of getting out of a headlock is to step forward with your stronger leg and throw your weight towards it, your attacker should tumble over ( iv done this to people who weigh much more than me and it worked)