part-time means it has no center diff. the prop shafts to front and rear are locked.
when driving, when you making a turn, not only the wheels on the outside is turning faster than the ones on the inside, the front wheels are turning faster than the rear cuz front wheels have a bigger turning radius. by locking the prop shafts, theres no speed difference and thats why when you turn tight, the whole thing would hop and jump.
full-time 4x4's like Subaru's have a center diff which let the prop shafts turning at different speeds when making a tight turn.
part-time is used only when the drive wheels lose grip, full-time is always in 4WD mode.
however with full-time unless the vehicle has very good limited slip diffs all round, its still can get stuck if the front to back grip has a huge difference.