OLL 40
The algorithm, how to recognise it and how to practise it without cheating.
Recommended algorithm
R' F R U R' U' F' U R9 moves
How to recognise it
A big lightning bolt: the left and right edges with the back-left and front-right corners. The front-left corner's yellow faces left.
Its inverse is another case
Do the 40 algorithm backwards, R' U' F U R U' R' F' R, and you are solving 31.
That's why CubeDojo never scrambles 40 that way: it builds a random state with the case fixed, so you practise 40 and not 31.
Practise 40 now
Random-state scrambles, a timer and spaced review. Free, on the web and on your phone.