The higher your feet are in relation to your centre line the harder it is and the more resistance you cummulate through the motion. the lower your feet in relation to your center the easier it is .. ( incline pushup)
Once you get the hang of that, if you want to raise your intensity again, you can roll down into the first part of the movement and stretch up elongating your spine through the lift in the second ..
these are called hindu pushups .. or court exercises..
besides the additional resistance of ending the exercise at a lower lever point for the second part of the motion, through the rolling and stretch follow up, you also get to gradually incorperate more muscle groups, work deeper and get a more efficeint range of work. If you can imagaine a lion and how he might approach a lake to drink from, you may be able to visualise some of the dynamics behind how they build such powerful haunches..
they are extremely difficult but for a level one version of that, you can try them starting with your knees on the chair instead.
Blooming lotus.