Culled from my ATPL theory notes:
An IVSI contains accelerometers mounted in the vertical axis connected to pistons. These pistons give an instantaneous pressure increase (descent) or decrease (climb) in the static supply before the choke as soon as a vertical acceleration is sensed by the accelerometer. This causes the VSI needle to move immediately upon climbing /descending, removing the large instrument lag with an ordinary VSI. Once the IVSI has hit the steady state in the climb/descent, it behaves just like an ordinary VSI measuring the pressure differential across a choke in the static line.
As with all ATPL stuff, don't know how closely this relates to real life, regarding the mention of a link to the IRS mentioned above
(typos corrected)
[This message has been edited by foghorn (edited 16 December 2000).]