Feet up, chest down, and your upper pecs light up.
Keep your hips from sagging, the higher your feet the more your upper chest does the work.
Elevating your feet tilts the pressing line upward, shifting the emphasis of the push-up onto the upper chest, with the shoulders taking a bigger share than a flat push-up and the triceps finishing the lockout. It also increases the portion of your bodyweight your arms carry, so it is both a regional shift and a straight progression. It is the closest a bare floor gets to incline pressing.
Yes. Elevating the feet changes the pressing angle so the load shifts up the chest, much like an incline bench angle biases the upper pecs. The higher the feet, the stronger the effect.
A bench or box around knee height is the sweet spot for most people. Go much higher and the movement drifts toward a shoulder press and gets far harder to keep strict.
The elevated feet make the plank part harder too. Squeeze your glutes, brace your core, and if the straight line still breaks, drop to a lower surface until it holds.
