Frank,
The two-pronged hook for Phil's arrangement has an inner radius and looks like it might be spring loaded. When it hits the lift ring (or wire choker or whatever) the spacer is knocked loose and the jaws close. Under load they should be self locking, which seems like a good thing.
The male latch pin on the HBOI is also a positive lock. It rattles down into the release funnel and once dropped into place, the spring loaded ears latch the unit into a lift ring. Once you've got the sub, then you've got the sub, period. It won't under any normal circumstance pull loose.