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.
The ability to remove delegation is in my own hands.
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.