Wot about the difference between a total energy variometer and a VSI?
The VSI measures how fast you are going up or down. The total energy vario compensates for speed changes.
For example you are gliding in still air. If you pull the stick back you will reduce your rate of descent and a VSI would show it. However you are trading speed for height. Although your VSI may show that you are climbing you are not actually gaining any energy, as you would in rising air, all you are doing is converting kinetic energy into potential energy (height).
This is not much use for the glider pilot so the total energy vario takes inputs from pitot as well as static to give the pilot an indication of total energy rather than rate of climb/descent.
In a typical scenario the pilot is flying through sinking air and has increased his speed so as to spend as little time in it as possible. He hits a thermal and immediately slows down and circles to take advantage of it. During the slowing down he will be converting speed to height. Without compensation a simple VSI would show the thermal as being much stronger than it actually is. It wouldn't show the true position until he had settled at a constant speed, at which point he might find that the lift was not as strong as he thought and not worth staying in. Total energy compensation works to eliminate this effect.
Mike