From a post by Matthew Ross: http://spacefellowship.com/Forum/viewtopic.php?p=38466#p38466
"Both of those were "LLC-style" hops where the mod flies gently up to about 55 meters and then gently back down.
Since both of those went well, we decided to do a "boosted hop," where instead of gently flying up and down, it goes full throttle for about three seconds, coasts to apogee at low throttle, falls quickly back down and then throttles up before touching down"
A 5 second serch on Wiki: http://en.wikipedia.org/wiki/Closed_loop
"A closed-loop control system is one in which an input forcing function is determined in part by the system response. The measured response of a physical system is compared with a desired response. The difference between these two responses initiates actions that will result in the actual response of the system to approach the desired response."
So, engine generates thrust X, desired target of which is X+Y. Throttle is increased until measured response is X+Y. At which point the throttle is maintained or decreased, depending on what part of the flight profile the vehicle is in.