You must be talking about a variable pitch propeller. A propeller you control blade angle the above is correct. At takeoff you also have dense air. If you had the high blade angle you would over torque the engine. As you get to altitude the air is less desne and requires a greater blade angle to maintain altitude and speed. In a constant speed prop, as the air gets less dense the prop wants to overspeed, hence increaseing blade angle to maintain the onspeed. Since you control the blade angle you actually increase the blasde angle at altitude to maintain the RPM (ie 100%).