The present invention relates to a method and device (1) for releasing into the body from a delivery system a medical prosthesis mounted on the delivery system and held by a constraint (16) in a constrained delivery disposition. The device comprises a first abutment (28) for the delivery system, a second abutment (18) for an elongate element (30) to connect the device to the prosthesis constraint, a track (14) for the second abutment (18) to advance along, from a starting point corresponding to constraint of the prosthesis, to a finishing point corresponding to separation of the prosthesis and constraint (16), ratchet means (38) to advance the second abutment (18) progressively, from the starting point to the finishing point, in a plurality of actuation strokes, and is characterised by a full stroke actuator (24), to advance the second abutment (18) all the way from an intermediate point on said track (14) to said finishing point in one single stroke of the said actuator (24), the intermediate point being selectable by the user within a portion of the track (14) which extends over at least half the length of the track.