@parus
Yes, it is. I've had it lock closed and open. It feels like the jaw is 'off its tracks.'
A couple of times I had to go to the dentist to have him put it back on 'the hinge.'
For a quick fix (that sometimes helped), I'd use warm compresses then massage until my jaw would go back into place.
Things like trying not to chew 'chewy' items like a chewy piece of steak or chewing gum can help.
The trick is to try not to overwork your jaw and to be aware of what you are eating (chewing).
If you hear clicking, that could be a sign that you are eating the wrong thing.

Ronnie (GRANDMAr)