Nucleophilic additioc of a secondary amine to the ketone or aldehyde, followed by proton transfer from nitermediate carbinolamine in the normal way.
Protonation of the hydroxyl by acid catalyst converts it into a better leaving group.
Elimination of water by the lone-pair electrons on nitrogen then yields an intermediate iminium ion.
Loss of a proton from the alpha carbon atom yields the enamine product and regenerates the acid catalyst.