The rules are:
1. The last key must be pressed AT LEAST 80 ms later than the second.
2. The last key must be pressed MAXIMALLY 240 ms.
A backspace (DEL) is generated with middle and index finger.
While pressing middle and index finger simultaneously (or longer than 240 ms)
gives an "l", you get a backspace when you first press the middle finger (as
long as you like, but at least 80 ms), then make a short tap with the index
finger and immediately release both fingers.