Timing and the more potential energy of the alcohol in the fuel. Ethanol runs cooler, resists detonation, thus giving you the ability to run more timing.
Think of a N/A motor. If you add more timing you usually get more power to a point. That point is limited by detonation. Add better fuel, the threshold for detonation becomes higher. You are able to run more timing, which makes more power.
Now switch to back to a boosted motor. If the boost is the same and you add more timing, you will make more power to the point of detonation. Add better fuel, you can actually lower boost (tuners do this to keep the fuel pressure stable esp with ethanol) because you can add so much more timing. That will give you more power while running less boost.