instantaneous VSI is found in most modern aircraft since any other VSI won't show you little changes in altitude btw. lag behind. All of our company helicopters, even those built in the 70's, are fitted with these kind of VSI's The following picture shows a schematic drawing of an instantaneous VSI:
Number 1 in the picture is the acceleration pump, number 2 are capillary holes. Acceleration pumps rely on the inertia of mass. When the aircraft starts to climb or descent the VSI moves upward or downward along with the rest of the A/C. The weight inside the acceleration pump that is hinged to a spring wants to stay at its place relative to space. Therefore the weight will move downwards when the A/c starts a climb. The higher the climb/sink rate the further the weight will move. Alike your body in your car when you hit the brakes - it woud like to keep on moving forwards. As a result a relative small amount of air is sucked out of the diaphragm unit on the left. The indicator needle will move upwards and show a positive rate of climb due to the lower static pressure inside the static pressure lines behind the capillary holes. The diaphragm unit expands or contracts due to pressure differences between the VSI housing and the static pressure lines. A mechanism is directly hinged to the indicator needle. The capillary holes are needed to compensate the pressure difference and slowly adjust to the correct static pressure. Since the VSI has to be a closed system in order for the diaphragm unit to work properly the capillary hole on the left will compemsate the static pressure difference before climb and after climb. In case of leveled flight the static pressure inside the VSI housing and the static pressure lines will be equal. In case of an ongoing continuous climb rate the acceleration pump willbe pulled back into its normal position by the spring and the diaphragm unit will do its work as in a regular VSI. All it does is show a pressure difference by expanding or contracting. A very simply but effective mechanical solution.
A regular VSI will lag behind due to the inertia of mass of the air. It will show a climbrate but it will start and end the process later.