The simplest way I can describe it is to think of a standard double-acting, spring-return-to-center hydraulic valve hooked up to a hydraulic cylinder. Hold the valve one way and the cylinder extends, hold it the other way and it retracts. If you were to "bump" the valve in either direction and let it return to center, the cylinder would briefly move in or out. If somebody specially calibrated the return spring for a certain rate, then they could roughly control the increment by which the cylinder moves its way in or out when you bump the lever.
That's exactly how the quarter-incher works -- it's a standard double-acting, spring-return-to-center hydraulic valve, but the springs have been chosen so that each bump of the lever between the "inching" stops moves the 3-pt hitch about 1/4 of an inch. If you hold the lever all the way in either direction it behaves normally and the hitch keeps moving. It's only when bumping the lever that you get the 1/4 inch increments.
The downside to the quarter-incher is that there is no correlation between lever and vertical position of the 3-pt hitch. You can move the 3-pt up or down, but you have to eyeball the height yourself. This is in contrast to position control, where you basically set the height directly using the scale on the lever, and it's repeatable every time. The other big difference is that the quarter-incher can drift down over time, and you'll periodically need to raise it manually. Position control has a feedback loop and will automatically adjust to maintain vertical position. If you have position control, you may have occasionally seen the hitch bump up as the feedback loop does its thing.
All 3-pts will float under the influence of gravity, at least until the lift arms reach their lower limit. To get float with position control, just push the lever to the lowest setting. To get it with a Kubota quarter-incher valve, you push the lever all the way down, past the lower stop and go into detent (the lever will stay there). With the Massey discussed here, it doesn't seem to have a true float position, so you either need to hold the lever down, or find a low spot and drop your implement all the way down there, so that you've found the lower limit of the 3-pt arms.