The QAD Ultrarest also uses the force of the arrow moving forward to drop. Although it uses a cord, the rest does not drop if you want to let the bow down.
HHA made one up until this year that was an Inertia rest. I wanted to try one, but since they are no more I have to settle for the Trapdoor. I just refuse to use a cord tied to my down-cable. I tried about 8 of them last year and I don't like them, period.
__________________
Martin Silver Star Shooter
UBP Life Member
PSAA Life Member
Worldwide Scuba Diver
The trap door works on vibration and inertia. Upon release the shock imparted on the riser causes the TD to fall. I had one for a coule years and never had a problem with it. I sold my old bow and the rest went with it. Haven't got around to getting another one yet.