To play passages in this register, the player shifts their hand out from behind the neck and flattens it out, using the side of the thumb to press down the string.
Electric bassists such as Brian Bromberg and Steve Bailey have applied the thumb position technique to their instruments because they share a common tuning.
In the jazz world, many bassists from the 1970s onward play both instruments, sometimes with equal proficiency (e.g. Stanley Clarke).
In slower, or more expressive playing, the contact point can move slightly away from the nail to the pad of the finger, allowing a fuller vibrato.
Thumb position can be, and is (by virtue of the requirements of the extensive repertoire) employed many times, and not only in the higher range of the instrument.