I definitely think being above/vertical has a lot to do with it. The d
rive ring that snaps on to the spokes has grooved and flat areas for the grooved belt to "drive" (the gaps allow it to idle, if the spoke ring had no gaps, the thing would take off when you cranked it.)
It sort of has the effect of "throwing you forward", if you know what I mean. My simile is a squirrel cage fan, throwing air out.
In other words, especially on level or declines, the belt, grabbing into those grooves on the
spoke/drive ring, is "thrusting the bike", like a push from behind.
On inclines, it does have more of a "pulling" feel, as if it is having to overcome weight and gravity. In most cases you can lean into a hill, and not have to assist until maybe halfway up, and in familiar territory, get a jump on major inclines.
The "trick" is
a: making sure the drive ring is snapped on perfectly centered on the spokes, which means that tension arm stays steady and doesn't jump or "act wobbly" and
b: making sure the belt seats centered on the gear attached to the drive shaft, doesn't rub too much on the sides of the gear.