How to play
A digit string appears for one second and is then replaced by dots. You type it from memory and press Enter. String length is set by difficulty — three digits at Beginner, up to eight at Expert — and the value never reappears, so there is no second look.
What you are actually training
This is working-memory practice as much as keypad practice. Most people can hold a short run of digits for a few seconds without effort and then hit a wall; for the majority that wall sits somewhere between five and seven digits, which is roughly where the Intermediate and Advanced settings land.
Chunking is the only reliable way through it. Read 4 7 2 9 1 3 as two groups of three rather than six separate items, and rehearse the groups silently while your hand moves. Speaking the digits under your breath during the display second also helps more than it sounds like it should.
A note on transfer
The reason this matters outside the game is source-document work: reading a figure from paper or a second screen and keying it without looking back. Every glance back costs a second or two, and those seconds are most of the difference between an average and a fast operator.