Having carried out a "Bump Start" on the fourth Griffon on a Mk II Shack several times, including at Gibraltar, it is quite straightforward, but you do not continue the takeoff!
At 80 kts PNF operates the Feather Button, to unfeather the prop. As soon as the engine is turning, fuel and ignition are introduced, the engine starts then PF closes the throttles and brings the aircraft to a halt. You allow for brake cooling, check for a mag drop, then perform a normal takeoff with all four running.