According to the invention, a back-end device (1) of an auto-injector (A) adapted to be attached to a packaged syringe (2) comprises- a plunger (1.3) to translate an injection needle (2.5) in a proximal direction (P),- a drive means (1.7) capable of, upon release, driving the plunger (1.3) in the proximal direction (P) towards a proximal position,- a rotary sleeve (1.2) with a cam surface (1.2.1) adapted to be engaged by a plunger follower (1.3.1) formed to the plunger (1.3),- a torsion spring (1.8) arranged to exert a torque upon the rotary sleeve (1.2) and- a release element (1.4) capable of, upon manual actuation, releasing the engagement of the plunger follower (1.3.1) with the cam surface (1.2.1). Upon release of the engagement of the plunger follower (1.3.1) with the cam surface (1.2.1), the rotary sleeve (1.2) is arranged to rotate within the chassis (1.1).The cam surface (1.2.1) re-engages the plunger follower (1.3.1) after a time delay determined by a rotation speed of the rotary sleeve (1.2) and is arranged to withdraw the plunger (1.3) from the proximal position in a distal direction (D) by continued rotation of the rotary sleeve (1.2).
