Regor,
My understanding of the floor mounted manual gear release hatch is simply:
When the hatch is opened an access door position switch sends a signal to the landing gear selector valve to prevent a hydraulic lock which may not permit manual extension of the landing gear.
If this access door is not properly closed during normal operations, the landing gear will not retract!
That is, the up side is depressurized to allow the free-fall of the gear!
Cheers,
