TES :
I'm with BlackSheep on this one - and it's not an IT question !!
The Vertical Speed Indicator shows a rate of climb or descent. This can be measured by seeing how quickly the pressure in a chamber changes when allowed to equalise back to ambient through a calibrated leak.
The choke has a capillary and a venturi part, all forming part of the same "hole" into the chamber. Because air viscosity changes with altitude and temperature, this arrangement automatically compensates. My Nav exam notes are being borrowed at the moment, so this is correct from memory.
An "IVSI" uses small dashpots, which induce an immediate change in the inflow or outflow of air from the chamber because they're basically weights on springs which move and push air in our pull air out of the chamber faster than would happen otherwise. They're there for use in aircraft which need an instantaneous ( hence the term ) VSI indication and not one affected by the small lag in display caused by the calibrated leak.