Fixing a mistake takes several times longer than typing the letter correctly: the mistake has to be noticed, the hand has to travel to Backspace and then you have to find your place again. A typist who makes three mistakes in a line is often slower than one who types calmly and cleanly.
This drill will not let you continue with a wrong letter. That is deliberate: typing is a matter of habit, and a wrong movement that is let through becomes ingrained just like a right one.
If one particular spot keeps going wrong, pick a pace at which that spot comes out right as well, and hold it for the whole line.
The drill uses long words, because that is where sloppiness shows. A short word can come out right by luck; a long one cannot.