Pianists often use finger substitution instinctively when they first attempt to play the organ. A free finger will naturally come to the aid of a busy one that needs to move somewhere else quickly.
Finger substitution is replacing one finger with another while holding a key down in order to free the first finger. It enjoys frequent use--sometimes too frequent--in organ playing, and results in a spider-like crawling from key to key.
It is notated with a short curved or straight line between finger numbers.
See the Finger Substitution video examples.