During a turn 2 things have to happen, 1)you need to unweigh the board and 2)you need to move your COM over the new edge. To do this you can do the cross-over or cross-under (theres alot of names for them, ie. down-unweighted for cross under). In the cross over turn, you are extending your body to unweigh the board and shifting your body over the new edge. In the cross under turn, you are sucking up your knees to unweigh the board and moving the board under you. When someone is doing the cross-over turn, you can see their upper body flexing and extending with maximal flexion after the turn completes, they then unweigh the board by extending to initiate the next turn. For the cross-under boarders, you see them flexing and extending at the opposite timings while their upper body stays more static than the cross-over turners. They extend during the turn and flex to unweigh the board to initiate the turn. So think about extending your leg through the turn and sucking them up to switch edges. Building up a good rhythm helps alot with these.
hope that helps