In ordinary text the hands take turns after almost every letter. One hand can line up its next movement while the other is still pressing, and that alternation is what makes typing fast.
Some words, however, fall entirely to one hand. Then there is no alternation and every movement has to be made by the same hand in a row.
left: database right: opinion
Such words come out slower and collect more mistakes. They are also the most honest measure you have: if one hand is less practised, it shows up here at once, because the other hand cannot help it.
When the gap is wide, drill the hand that lags rather than both equally. Usually it is the left one, and above all its ring and little finger.
The drill mixes both hands. Watch which of them holds you up more. For the opposite case, where the hands change over after almost every letter, see the next lesson.