I believe this is done with delegation, a feature not directly available through steemit.com. So he doesn't transfer STEEM, he merely delegates STEEM Power, delegation which he can remove at any point. See @minnowbooster for a more "public" service offering the same.

Beware that STEEM power cannot be traded/transferred until after a long period of "powe down". By leasing it out you can get liquid (as in transferable) STEEM out of it and still keep the power for your own use later.