Once you can perform Pike Push-ups with your hips directly above [...]
The next progression after decline push-ups towards the handstand push-up, is the pike push-up. In this exercise, we move the arms so that they are in a vertical pressing position, rather than the horizontal pressing position used in push-ups. To make it easier than a handstand push-up, we simply put some weight through the legs.
The first progression after full push-ups towards the handstand push-up, is the decline push-up. By elevating your feet, more weight will go through the upper body, which will condition your body ready for the next progression.
The first progression after full push-ups towards the one arm push-up is the close hand push-up. By moving your hands closer together, it puts more emphasis on your triceps, which will need to grow stronger in order to perform a one arm push-up.