2017-06-04 (Initial Post)
There are number of articles on what STEEM POWER is but hardly any one explains how it is transferred from one owner to another. The steemit.com wallet UI will tell you it's not transferable and you can find a number of similar statements in other web resources.
However, the fact that you can purchase STEEM POWER from BlockTrades means they have some method for transferring it to you. What is that method and is it accessible for the rest of the community? According to Reddit (https://www.reddit.com/r/btc/comments/4ssol4/opinions_on_steemit/) people may have tried selling their STEEM POWER by selling their private keys. This could be "super risky" and the post re-affirms that "you can't transfer Steem Power trustlessly by design" It also mentions that "the best you can do to sell your Steem Power is to negotiate a futures contract (which don't yet exist)"
So the question remains: how is Block Trades doing something that is supposed to be "impossible by design"?
2017-06-10 (Update)
Looking some more, I found the answer I was looking for ironically from Block Trades themselves https://steemit.com/steemit/@blocktrades/sending-steem-power-to-another-person-using-steemit It turns out there's a "power up & send" feature in both command line client and web UI. So in the process of powering up your steem you can also say that you want to transfer it (e.g the power it turns to) to another person.