The invention relates to crosslinkable or crosslinked silicone compositions capable of being used to form a water-repellent and release coating for a flexible support made of paper or of polymer, in particular a heat-sensitive flexible support. These compositions are of the type of those comprising crosslinking polyorganosiloxanes carrying SiH units and unsaturated, preferably vinylated, polyorganosiloxanes capable of reacting with the crosslinking agent by polyaddition in the presence of platinum to form the release crosslinked coating on the flexible support. The targeted aim is to make possible the instantaneous crosslinking at low temperature (85-90° C.) of such silicone compositions coated on flexible supports at a very high speed. To achieve this aim, the invention provides for the use of specific crosslinking agents composed of POSs hydrogenated at the chain end and in the chain. The invention also relates to the silicone compositions of solvent-free type or of emulsion type comprising the Si-Vi POS, the SiH POS crosslinking agent α,ω-bis(hydrodimethylsiloxyl)poly(hydromethyl)-(dimethyl)siloxane, and the platinum catalyst, inter alia. Applications: release silicone coatings obtained by crosslinking/polyaddition for heat-sensitive flexible supports, for example polymeric films made of polyethylene.