Hold the space bar down. A short press is a dit, a long one a dah, and the length of the silence after it decides where one letter ends and the next begins. Nothing is typed here: the only thing the tool reads is how long you held and how long you waited.
The register drew marks on a moving tape and the operator read the lengths off it. This one runs the same way, sixty units of tape wide, with a tick for every unit along the bottom. The bar you are still holding is coloured until you let go of it.
Morse has one length in it, the dit, and everything else is counted in dits. A dah is three. The gap inside a letter is one, between letters three, and between words seven. So the whole code is a rhythm rather than an alphabet, and two operators at different speeds send the same thing as long as the ratios hold. The speed dial sets how long a dit is in milliseconds — at twelve words a minute it is a tenth of a second — and everything else follows from it.
A press is read as a dah past two units, the midpoint of one and three. Silences are read against a second unit, because a hand does not pause the way it presses: the elements inside a character are muscle memory and come out at the speed of the dial, while the gap before the next character also holds however long it takes to remember what that character is. Counting both against one unit is what makes a beginner's message arrive as single letters — every thinking pause reads as the end of a word. The trade has had a name for the fix since the 1950s: Farnsworth timing, characters at one speed and the spacing at another, and the spacing dial is that second speed. Set it equal to the sending speed and this reduces exactly to strict timing.
The thresholds stay midpoints, which is still where a guess is least likely to be wrong: a silence ends the letter halfway between one character unit and three spacing units, and adds a word space at five spacing units, halfway between three and seven. Both are printed beside the dial in milliseconds, so what the tool is waiting for is never a guess on your part either. If your letters keep running together or breaking apart, the dials are set to somebody else's hands rather than yours; the reading beside them measures both your dits and your gaps, and the match button sets both.
A pattern with no character against it is printed in angle brackets the way an operator writes a prosign, so nothing is silently dropped. Backspace clears the letter you are part way through, or removes the last one if you are between letters.