Pick a class and a number of moves. The page shows a DR case as a scramble and as a cube you can drag around. Find a solution that reaches DR, then press Found it to stop the clock and check it against the optimal ones.
With Tap to turn on, tap the centre of a face to turn it clockwise. Your moves are listed under the cube, and reaching an optimal DR stops the timer and grades the answer for you. A longer DR keeps the clock running so you can reset and retry. Drag anywhere else to turn the view.
Spaced review shows cases that are due, then new ones, and brings a missed case back within a minute. Recap shows every case in the set exactly once.
Cases are every DR with an optimal length up to 6, one per shape. Mirrors and turns of U and D before the case count as the same case.