Buying Steem Power - how is this even possible?

I am a little bit confused, I thought it is not possible to trade "Steem Power" but on Blocktrades.us it is possible to buy SteemPower. How is this even possible?

I thought we have 2 liquid assets (Steem and Steem Dollars) and one none tradeable class (Steem Power). But when the transfer of SteemPower is not possible how can Blocktrades.us do it?

This is really confusing, so maybe someone can share some light on it @ned @dan @blocktrades