The "Choking off" mechanism takes place after topical application of:

When sodium fluoride solution is applied on the tooth surface as a topical agent, it reacts with the hydroxyapatite crystals in enamel to form calcium fluoride, which is the main end product of the reaction.  As a thick layer of calcium fluoride gets formed, it interferes with the further diffusion of fluoride from the topical fluoride solution to react with hydroxyapatite and blocks further entry of fluoride ions. This sudden stoppage of the entry of fluoride is termed as the "Choking off effect". Fluoride then slowly leaches from the calcium fluoride. Thus, calcium fluoride acts as a reservoir for fluoride release. It is for this reason that after each application of sodium fluoride onto the tooth surface, it is left to dry for 4 minutes. Essentials of preventive and community dentistry  Soben Peter  5th edition