A vertical speed indicator uses the static port pressure as an input, and uses mechanical methods to differentiate it with respect to time (via the use of a calibrated leakor other means, or electronically). A vertical speed indicator (or "uncompensated variometer") is rarely used in gliding because it is not able to show the increase or decrease of the total energy which is more important than the absolute climb or descent, as glider pilots frequently trade height for speed and the other way. If you trade your speed for altitude a vertical speed indicator will show that you are climbing, but in fact you are losing total energy slightly, as gliders do all the timeso often that it has got its own name "stick thermal". For this reason, a vertical speed indicator is not particularly good in the identification of lift sources in soaring.