There are 2 types of real "lock jaw" out there though:
1. An illness called Tetanus (http://www.seefido.com/canine-dog-health/html/dog_tetanus.htm)
2. A comic book character "An Inhuman transformed into the shape of a dog by the terrigen Mists" (http://www.comicvine.com/lockjaw/29-3534/)
So unless you want to give your dog tetanus or make him hang out in the terrigen myths, I wouldn't "train your dog to lock its jaw"
I have owned Pit bulls and other terriers. I have had this discussion with my best friend who also owns pit bulls.
We exhausted every avenue of "lock-jaw". Read every thing we could find about lock jaw and asked numerous Vets.
They all had the same answer......No dog on this earth has "lock-jaw". The only case of a dog having lock-jaw was a dog with tetanus lock-jaw, it is very rare that a dog gets this type of disease , that is the only type of lock-jaw a dog may get. It's a medical condition , nothing to do with the actual "lock-jaw"
I hope this answered your question...there is no such thing as a dog with "lock-jaw". Its a myth.
Check out this site, its the one we found to be most creditable and he confirmed the "lock-jaw" myth
http://pitstopnc.tripod.com/id2.html