As discussed in the SCALE/ARPEGGIO -TYPE SHIFT section, finger substitutions are an intermediate phase along the continuum that exists between a semitone hand contraction and a fully fledged scale/arpeggio-type shift.
WHY (AND WHEN) DO WE USE FINGER SUBSTITUTIONS?
It is very hard to disguise (camouflage, hide) shifts that occur during slurs. Therefore, in passages that want to be played “cleanly”, in order to ensure that there are no ugly “smears” (position changes with glissandi) on the slurs, we usually try to do the shifts before or after the slurs. This often involves using finger substitutions. Substitutions are a great way of hiding shifts as it is normal – and easy – to do them without any glissando.
This ability of finger substitutions to avoid glissandi is why we use them more often in clean fast passages (where we don’t normally want expressive glissando effects) than in slow Romantic or lyrically expressive music (in which we actually want the expressive glissando effects).
A comparison of Piazzolla’s slow beautiful lyrical SIN RUMBO with some passages from Rossini’s Duetto for Cello and Bass gives us many more examples of this. In the Piazzolla piece we definitely do not want to use finger substitutions, because the glissandi shifts are beautiful, and are actually necessary for the music’s expressivity. If we were to use finger substitutions in this piece, the music would sound dry and unexpressive.
In Rossini’s Duetto on the other hand, those “beautiful” Romantic glissandi would be completely out of style and would sound positively ugly. Therefore in Rossini we definitely prefer to use finger substitution fingerings.
In the Piazzolla example, the red rectangles indicate position changes on which we can choose to do beautiful juicy romantic glissandi (instead of nice clean unnoticeable finger substitutions).