When the hands are held over the key-board in the position for writirg, it will be seen that the thumb is in position for striking the space-bar; and by using the thumb for this purpose instead of the second or third finger, as some do, the time occupied in carrying the hand down, and returning it, is saved, and the spacing is easily done with the thumb. （page. 4)
Elizabeth Margaret Vater Longley Type-Writer Lessons for the Use of Teachers and Learners Adapted to Remington’s Perfected Type-Writers』(Cincinnati, 1882年)
Without caution like she informed, people might be apt to use other than thumbs.Why were they apt to touch space-bar by index or middle or third finger?
The revolutionary participation of thumb(s) might lead to touch.
"the thumb will be in readiness to strike the space-bar"
Mrs. M. V. Longley: ``Writing Machines'', Proceedings of the First Annual International Congress of Shorthand Writers, pp.14-16 (August 31, 1882).
If waiting at the position the thumb is in readiness to strike the space-bar..
The thumb's readiness to strike the space-bar hides some key-tops under the hand.
So while taking the complete site method, people doesn't want to strike space-bar by thumb.
To strike the space-bar by thumb needs partial memorization of key-layout at least.
This consists also on little finger key operation.
If operating keys on outer side of the keyboard by little finger or ring finger, key-tops under forefinger side are hidden.