Appropriately named, during armstand dives the diver executes a dive from the armstand position. Armstand dives are one of the six groups of dives, and are only used in platform diving.
The armstand can be executed from either the forward or backward position on the platform, and the manner in which the diver initiates, and/or maneuvers to the armstand position, is not to be a factor in how the dive is judged.
According to FINA Rules the starting position of an armstand dive (and the moment when judging of the dive begins) is when both hands are on the end of the platform and both feet are off the platform.

