Refer to figure.
The Vertical Speed Indicator (VSI) measures the differential pressure between the actual static pressure and the previous static pressure. However, this indication is not immediate and requires several seconds of adjusting - being reliable only when flight is stable. In order to solve this lag error, an instantaneous VSI (IVSI) unit is used which will consist of the same basic elements of a conventional VSI but also employs an accelerometer, designed to create a faster differential pressure effect (especially at the start of a climb or descent).

The IVSI has 2 accelerometers fitted in the static pressure line => as the aircraft manoeuvres, the load factor changes and the accelerometers will move artificially, changing the pressure in the line to overcome lag. The response time of a VSI is, therefore, reduced (making it an IVSI), by fitting of dashpots into the static feed. These dashpots centred with springs sense a pitch change/vertical acceleration and alter the pressure in the static feed line => which reduces the time to react.