Yes, that’s it. It is a full keyboard, silent, which allows you to vary the key resistance. Mine goes up to 20 ounces but I use it on around eight ounces at the most. The idea, fairly obviously, is to strengthen the hands and fingers to a point beyond that required for the heaviest piano action. They are decidedly unfashionable these days in musical circles, with most pianists never having heard of them. My teacher in my youth, a skilled cabinetmaker, rebuilt one and sold it to me in 1968.
Admittedly, thoughtless use of one can do more harm than good, and it takes quite a while to work out exactly how to play it to maximum benefit. I have used mine for a few minutes a day most of my life and I am sure I would not be playing the sort of stuff I do at seventy-one without it. Although I do have a small repertoire, I am primarily an improviser and, once at the instrument, all is music. So it is true that, aside from technique on the Virgil, I do virtually no practice in the usual sense of that word.
I do not suggest they are right for everybody, but mine has proved invaluable for me.