When Gary says "5 degrees angle of attack," imagine the airplane moving along horizontally, but with the nose pointed up 5 degrees. The airplane is actually flying in one direction, but is pointing in another direction.

Because of the twist in the propeller, the blade moving down will "dig" into the air more than the blade moving up, when the airplane is flying along in this situation. On a normal propeller, that downward-moving blade is on the right side of the airplane.

The downward-moving blade produces more thrust, and more drag as Gary mentioned. Extra thrust on the right side makes the airplane want to turn left; this is why taildraggers tend to turn left when taxiing and taking off.