it's impossible sit straight and to move your bod forward without your seat tilting forward... follow his neck only with your hands, not your whole body. you have three standard positions in the canter; sitting, two point and half seat. what @ameliaduplooy
is talking about is a half seat. but a half seat is to get off your horses back and give him some freedom, to slow him down you'd block with your seat. or if you need to give your horse more balance and you're in a half seat just open up in your upper body.