Troweling is a technique by itself. If you just pour concrete and zoom in it will have waves (a smaller scale like broom finished). That's why you cant see your reflection. So the answer is at the perfect time go over it and you cut the waves in half and it gets smoother. If you do this right you will end up with a mirror finish. But it do it too long you can burn the surface black. So the only answer is perfect timing and go over the whole piece and let it rest.