The single most important factor in string crossings is preparation, the basis of all technical fluency. Although a little extra bow speed for shifts and string crossings can be helpful, the budget-conscious cellist cannot always afford such luxury over some of the longer slurs. In staccato, work for an equal bite and amount of bow on each note. Prepare all string crossings. The greatest benefit in practicing Popper is, of course, achieving ease, comfort , and, eventually, mastery over the entire scope of the fingerboard.
Popper is the anthology nonpareil for establishing a secure thumb position.
In the time it takes to move from one position to the next, reconfigure the hand so that, upon arrival, the new village is entirely laid out, thumb included. The thumb is an integral part of the hand at all times and must be accounted for. Create a temporary alternate anchor on a 3rd or 2nd finger, while moving the thumb quickly and efficiently across from its old to new string pairing. In the rest of the book, however, use of the harmonic is far too liberal. Otherwise, a solid finger will better establish and clarify the new village.