One of the factors that determines the efficiency or otherwise of a glider tug is the rate at which it can get down again after dropping the tow - the quicker down, the more launches it can carry out during the day. Unfortunately, closing the throttle and hurtling earthwards causes rapid cooling of an air-cooled engine, and if carried out as a matter of routine will give rise to a high rate of cracked cylinders. We found that out at Dunstable; the solution was to keep plenty of power on during the descent, although this delayed the return for the next launch.
A turboprop or water-cooled engine doesn't have the same problem. The Wilga has, I believe, an arrangement of shutters in front of the engine that can reduce the air flow through the engine during the descent.