A vertical speed indicator (VSI) is an instrument, which displays the rate of climb or descent of the object to which it is attached, commonly an aircraft. There are two types of VSIs. The most common and simplest is a barometric VSI, which displays the apparent vertical speed of the aircraft as is detected by a change in static pressure, from the static port. The second type is an instantaneous VSI, which combines accelerometric readings with barometric sources . Its good to know if you are falling out of the sky, but more usefully and commonly used for instrument (IFR) flying.