Found from my ATPL notes also:
IVSI( also known as an inertia lead VSI)is similar to a VSI but incorporates a dashpot(cylinder) which is mounted on a spring loaded piston. Upon say entering a climb, the vertical acceleration forces the piston down the dashpot and in doing so sucks air out of the capsule. This, in turn makes the capsule contract, which moves the VSI needle to show a climb. The reverse happens should a descent be iniated. The normal VSI sensing system eventually catches up and continues to show the climb or descent as would a conventional VSI.