The Throttlemeister works by providing friction against the outer edge of the throttle-sleeve, keeping the sleeve from turning on its own. It's 'smooth', metal-to-rubber flat-surfaces contact with nothing to get stuck, or locked hard, i.e., you can always overcome it by twisting the throttle grip. To your questions of providing a 'little too much gas', since it's passive, it won't increase the 'gas' unless you make it do so. As for getting stuck such that you can't turn the throttle, you wouldn't be able to twist the throttle in the first place, before you'd get to that point.
You twist the Throttlemeister to engage it. Then, you twist it the opposite direction to disengage. When engaged, the mechanism applies enough pressure to the outside edge of the throttle sleeve to overcome the return spring's force, thus holding the throttle in a fixed position. On a steadily inclined (effecitvely 'flat') roadway, you'll make occassional, small twisting adjustments to throttle position to hold a steady speed. These adjust for slight variations in wind and/or surface inclination. More pronounced inclination deltas will call for greater adjustments. You won't use this in town, on surface streets, nor in heavy traffic (at least, I don't). It's more for the open road, of course.
You're provided a number of washers of various thicknesses with the installation kit. You mix-and-match these washers, through trial and error, to adjust the tolerance of the Throttlemeister such that, when it's disengaged the throttle turns easily, but when it's engaged, the throttle is harder to turn--and won't turn by itself. You want to adjust it to 'just' allow the throttle to turn freely, when disengaged.
In my case, on very cold days (25* F ambient, or less) and when moving at 70 MPH, the Throttlemeister 'slips', i.e., won't hold the throttle position. I assume this is because in such cold temperatures, everything contracts and the Throttlemeister doesn't apply as much force against the end of the throttle sleeve as it does when things are warmer. This is not really an issue because even in this state, the Throttlemeister applies enough friction against the throttle the slow-down is very gradual, allowing me to remove my hand from the throttle to rest it. I mention this to point up the fact that temperature will have an affect on the T-Meister's tolerances, and so its adjustment. I live in the desert (temps vary from 110*F in summer to 17* F in winter). I have a 'summer' adjustement (set of washers) and a 'winter' adjustment (a different set of washers). If the climate in your area is not so extreme as this, you could reasonably have a single adjustment that works year-round.
I literally couldn't ride my bike long distances without the Throttlemeister, and the 'rest' it affords my right hand. I've had them on several of my bikes, and anymore, it's one of the first things I buy for any bike I own that doesn't have an integrated cruise-control.
alan