Need to know what it does

It stops channeling the spell, and goes to the next action in the api.
Use:
actions.vf+=/mind_flay,if=!talent.mind_spike.enabled,chain=1,interrupt_immediate=1,interrupt_if=action.void_bolt.usable
It means if you are channeling mind flay and void bolt "becomes" usable stop channeling immediately and cast void bolt.