The invention concerns crosslinkable silicone compositions, useful for forming water-repellent and release coating for flexible heat-sensitive support. Said compositions comprise crosslinking polyorganosiloxanes (POS) bearing .ident.Si--H units and unsaturated POS, for example .ident.Si-Vi vinyl-containing POS, capable of reacting with the crosslinking agent by polyaddition, in the presence of platinum to form a release crosslinked coat on the flexible support. The invention aims at instantaneously enabling low platinum content crosslinking of such silicone compositions, coated at high speed. Therefor, the invention provides for the use of particular crosslinking agents consisting of hydrogenated POS comprising at least three hydrogen atoms bound to the silicon located at chain end and in the chain. The invention also concerns solvent-free or emulsion-type silicone compositions comprising the .ident.Si-Vi POS (I.1), the crosslinking agent .ident.Sl-H POS (l.2) and the platinum catalyst among others. The invention is useful for coating flexible supports, for example made of Lisse BEKK>1000 paper.