The usual procedure to deploy a german fallschirmjaeger´s gravity knife is the following:
unlock the blade
make a whipping motion with your hand (most of it comes from the wrist)
release the locking lever with the proper timing to lock the deployed blade
fold the lever to the front to prevent accidtial unlocking

Inertia forces work much more reliable then gravity and are almost as fast as a swichblade. My main complaint about it was the lack of lateral stability of the blade.
The blade was only meant as a tool. The spike was the weapon. It leaves nasty wounds.
BTW: We where not supposed to deploy a gravity knife or an automatic knife in plain view of an injured person.