Usually you attach one end on the carb linkage and the other on one of the engine holding screws. If you linkage comes undone the spring will save the day but if your battery goes dead or disconnected it wont. The spring wont be strong enough to return the servo back and you cant use a very stong spring that would do it as your servo wont last long.
It is really so simple that there isnt really any point for a company to market something like that.