This is a very common 3D maneuver. Basically its entered by going to vertical and having the plane stop its assent, at this point, power is applied to stop the plane from descending. Full down elevator is applied causing the plane to rotate around its own axis as it falls. As with most maneuvers, there are variations. You can do the Waterfall positive or negative, you can impart a spin to it and also increase power when the nose is up causing the plane to maintain altitude. The exit of this maneuver is interesting and a place where to me many ruin the maneuver. The common exit is to time opposite elevator input stop the rotation and then fly out. I find this to be less than ideal and not very precise.

An alternative is to exit the maneuver rolling, i.e. use the rudder and ailerons to exit the side of the maneuver in a full stall roll. This really doesn't require timing and looks very smooth.