An open differential is a triangle of rotating shafts. Hold any one and the other 2 turn. When not properly engaged the "free" shaft turns because it has the least resistance.
It isn't the fork or pin that breaks often, but the sliding splined coupler or its associated gears. You may not know until it is opened and inspected. Watch out for gear oil leakage out the side when you open it.
This might make it a little clearer:
https://www.youtube.com/watch?v=ZtC52KShPR0