Drive a knee to your elbow mid push-up and your obliques crash the chest party.
Keep your hips level as the knee comes up, do not let the dome rock or your back sag.
The push-up loads your chest through a full pressing range on the unstable dome, then the knee sweeping to the elbow mid-press drops one foot from your base right when the chest is working hardest. Your triceps and shoulders steady the lockout while the platform tries to rock with the moving leg. It earns a place as a push-up that trains pressing and hip mobility in the same rep, with the dome punishing any sloppy timing.
As you press up from the bottom, the knee drives out toward the same-side elbow, then the foot returns before the next descent. Timing it with the press is what keeps the rep smooth instead of a stagger.
You lose a foot of support mid-rep while pressing on a surface that already moves. Three points of contact on an unstable base means your whole trunk works overtime to stop the rock.
Yes, alternate knees each rep so both sides get equal work. If one side consistently rocks the dome more, that is your weaker side telling on itself.
