Just to confuse the issue, an IVSI (where the first "I" is Inertial not Instantaneous as many think) have masses that, as the aeroplane accelerates vertically, cause displacement of air and thus a reading on the dial. Can't tell you much more than that on their operation without digging through the books (unfortunately having a great time working on a tax issue) but I believe this was to counter the altitude related inaccuracy. Most glass aeroplanes grab this info from IRS equipment - far more accurate and not susceptible to variations in the atmosphere.