1) Take a fluid, color it mostly white(ish).
2) Suspend a bunch of tiny black "balls" inside this fluid. The balls need to be charged so they can be attracted to an electric field.
3) Squish this fluid between some glass.
4) Put a grid of electrodes on the back that you can address.
5) Now, using some clever AC patterns, you can address any "pixel" on your grid, and apply a charge there, that will attract or repel the little balls.
6) When you attract the balls, that spot turns black, because you just see the balls above the fluid.
7) When you repel the balls, that spot turns "white" because you just see the fluid, not the balls.
The trick is that moving the balls back and forth (6/7) is a physical process, and requires time. They _have_ to move a physical distance, and if that fluid lets them move too easily then they won't keep their color. The balls will move and they'll just drift back to some middle ground.
Also, if you're not careful with your approach, you can mess with the charge of the balls, and they'll no longer respond to the field the way you want. This is burn-in. (And it's why displays do full-black-to-full-white refreshes from time to time)
So until we can figure out how to move those balls faster in the fluid, we really aren't going to dramatically improve refresh rates with this technology.
Note: "Balls" is a simplification.