So - keyswitch is used for some synths, to have the base sounds for a given preset be able to be modified without having to load different presets.
So, using Kontakt as an example, with their VSL Strings preset loaded, playing most notes results in normal strings sounds, but if certain notes (key switch notes) are pressed, each one of those kinds of notes would alter the strings sound in some manner. One key switch note for the VSL Strings preset changes the strings sound from legato to pizzicato (plucked). Another changes the sound to a sharper attack, simulating a different bowing technique, etc....
Once a key switch note is played, the sound will remain altered per that key switch note's attributes, until another key switch note is played.
For my use, I lowered the note numbers for each of the note switches for the VSL Strings preset by an octave, to expand the range of notes playable for the base sound, without accidentally changing the sound by a key switch. (I use love octave string notes frequently and I kept hitting low notes that would change the sound attributes without my wanting the sound changed, so lowering the key switch note numbers by an octave gave me an additional octave of low string notes that could be played).
I hope the above explains the concept of key switch notes.
And Glyn Barnes also posted 2 great ideas:
"A drum map can be used as it lets you assign new names to notes in the PRV - So instead of seeing C0 C#0 D0 etc you would see legato, stacato, trill etc.
Note that in some cases you may want to move your key switch notes ahead of the beat to make sure the articulation changes before the note starts to play." Bob Bone