It is the 'bendix' gear on the starter motor that is causing the jamming. As the starter spins quickly, the bendix gear is shot up into the flywheel which it spins, hopefully starting the motor. Once the motor has spun, the bendix gear then returns to its rest position, ready for the next time. I think your starter motor isn't spinning fast enough; give it another try with a fully charged battery. If it still locks up, replace the bendix gear.
John, May 2008