What the inerter does is it gives the feeling of greater inertial mass between the two ends of the device. If you push on the inerter it feels like you are pus

What the inerter does is it gives the feeling of greater inertial mass between the two ends of the device. If you push on the inerter it feels like you are pushing on a boat in a marina: slow go get moving when you push it, and slow to come to rest when you push (or pull) it back the other way. It does this because flywheels hold momentum but as angular momentum. The inerter converts linear motion to angular momentum. The gearing also it to give the same effect of an enormous inertial mass without itself being massive. My understanding as to why this is useful is as follows. Suspension works best when the vehicle is heavy and the wheels are light. Imagine a battle tank on ultra strong bicycle wheels: the tank would glide along as the wheels jump up and down over every dip and bump. Something similar happens when you drive home with a car load of paving slabs. Having a massive vehicle is a huge penalty though for other reasons. Battle tanks are not nimble! The only thing you really want the mass for is as an inertial mass in the suspension system. For accelerating and going around corners you’d rather it wasn’t there at all.