Grips, per se, aren’t really relevant to keyboard use, but would be relevant for mice, knobs, sliders, throttles, joysticks, pens, and similar devices. The distinction between the “precision” and “power” grips is not the direction of thumb movement. Grips are about opposition. Opposition between thumb and fingertips is a “precision” grip, whereas opposition using the palm is a “power” grip. A precision grip is how you hold chopsticks, a paintbrush, a fencing foil, a chef’s knife, etc., and by combining arm, hand, and finger movements, it’s possible to make very accurate tiny movements. A power grip is how you hold a cleaver, a large hammer, a baseball bat, etc., where you mainly need strength and large arm movements. When you open a stuck glass jar lid, you switch from a power grip to initially unstick the lid, and then finish unscrewing it with a precision grip.
Anyhow, slap bass is a somewhat different kind of motion from pressing a spacebar. Slap bass players keep their thumbs mostly rigid, and rotate their forearm to get the slap. It’s possible to use forearm rotation to press a spacebar but in practice most people type space by abducting (and maybe slightly flexing) their thumbs.