The pin press is like a bottom-up bench press. The lift begins from off the chest.
The pins or safety supports can be set one inch off the chest, in the middle of the range of motion, or high up near lockout to strengthen any sticking point you have. Remember, don't bounce the bar off the pins as you do reps. Let the bar "fall dead" by pausing a second or two and resetting each rep.
Pin presses also work great with supramaximal training – lifting from the sticking point of a lift with weight greater than your max. This is possible because you're using a partial rep.