Hang Spell is cast on another spell (cannot be a Blocking spell) to delay its activation until the caster wills it. Hang spell is cast immediately before the spell to be hung with no interruption in the casting times and "counts as one spell “on” for casting purposes; it does not, however, count against itself when it is activated."
Activation time[]
This is presumably a free action, meaning it would be done at the start of a turn, as opposed to Reflex which allows it to be done any time during a turn as an active defense in response to another action.