A wobbly dome turns a narrow push-up into a triceps and core grind.
Hands close means elbows stay tight to the body, that is what shifts the load onto your triceps.
A close grip on the flat side of the dome keeps your elbows tucked and shifts the pressing emphasis toward the inner chest and the lockout, where your triceps take on their biggest share of the work. Your shoulders steady the path while the narrow base makes the dome want to tip at the slightest imbalance. It earns a place when you want a harder push-up that builds pressing strength through the elbows without touching a weight.
A close hand position already shrinks your base of support, and the dome multiplies that by moving. Any uneven pressure tips the platform, so your pressing has to be perfectly symmetrical.
Triceps and chest, with the triceps doing noticeably more than a standard-width push-up. If your shoulders scream instead, your elbows are probably flaring, keep them brushing your ribs.
Yes. Get comfortable pressing on the wobbling platform at shoulder width first, then bring the hands in. Narrow grip on an unstable surface is the advanced version, not the starting point.
