For those using notation programs who are new to keyswitching, here's a visual aid which might help. Gary was kind enough to provide the link.
The keyswitch notes are merely notes you enter in your score which are out of the instruments normal range and produce no sound in GPO. They exist in the first complete octave starting on C beneath an instruments lowest note. The example uses the Violin 1 KS combo patch. By default the initial sound this patch uses is the sus/short bow. If a different patch is needed insert the appropriate keyswitch before the note sounds.
To switch to pizz. in bar 2 of the example, place the F keyswitch just prior to the downbeat of bar 2. You can put a keyswitch into your score while another note is sustaining.
Placing the C keyswitch just before the downbeat of bar 3 changes the sound back to arco.
Now to hide the keyswitches. These were done in Finale. In example 1 the keyswitch notes were placed in layer 4. After all of the keyswitch notes have been added you can use a staff style to hide layer 4. It'll playback correctly and won't be confusing to look at.
Another way to use keyswitches is shown in example 2. Put all of the keyswitches on a separate staff. As long as the keyswitch staff is on the same midi channel as the primary staff, it'll playback correctly. Once finished, in Finale use the staff attributes to hide the keyswitch staff from view.
I believe in Sibelius you can use a similiar approach by using separate voices and hiding the keyswitch voice.
Hope this helps a little.