I learned from the LIGO official website that the LIGO mirror suspension system consists of a "main chain" and a "reaction chain", and there are small electric motors gently pushing the masses on the main chain (i.e., the mirror and other suspension masses) against their counterparts on the reaction chain to "keep them in place". Here is a link to the material (see the second paragraph under the schematics of the suspension system).
I am wondering why this is a good strategy to achieve better isolation. It appears to me that the whole point of isolation is to keep the masses free (at least in some frequency band and along some direction), and the idea of installing motors seems to be ruining it. Also, I do not understand how these motors are controlled to distinguish between unwanted disturbances which should be corrected for, and true gravitational wave signals which should not be corrected for.