Inside the IVSI, in addition to the normal plumbing, is a weighted valve of some kind...it's suspended by springs in its neutral position inside a tube. The "top" side of the tube is ported into the plumbing of the IVSI, and the "bottom" side is open to the cabin. This setup allows the inertia of the pull-up or push-over to affect the VSI indications.
Maybe if you imagine a syringe, with the injection end up, and plumbed into the VSI, and the plunger suspended with the springs.
When the airplane begins a descent, the mass in this valve initially moves upward within the instrument (just like when you get light in the seat when you push over into a descent). The way it's plumbed, the weight then pushes a small amount of air into the instrument, simulating an increase in static pressure, resulting in an "instantaneous" descent indication. Once the descent stabilizes, the springs allow the mass to move back to center, and the VSI indication is based solely on static pressure change.
Conversely, when the airplane begins a climb, the valve makes an initial movement downward, drawing air out of the VSI, indicating an initial climb. Again, when the climb stabilizes, and inertial forces are removed, the mass moves back to center, and climb rate indication is based solely on static pressure change.
An instrument shop checks the operation of an IVSI by flipping it upside-down, and noting a temporary climb indication.
Hope this helps!
David
