A pocket of liquid
At the heart of every die is a liquid core sealed inside solid resin. That liquid is what lets the eye move.
An eye that drifts
The eye floats in that liquid, so it shifts and turns as you roll and tilt the die. Buyers describe it as "trippy", and no two rolls look quite the same.
Best under a lamp
Hold a die up to the light and the resin glows around the eye. On a dark mat or in a dice tray the numbers catch the light too.