From Synfig Studio :: Documentation
Holding allows you to select more than one waypoint for a given valuenode at a time.
- When nothing is selected, clicking on a point in either normal mode or additive mode will select the time point closest to the click. Subtractive click will do nothing
- When things are already selected, clicking on a selected point does nothing (in both normal and add mode). Add mode clicking on an unselected point adds it to the set. Normal clicking on an unselected point will select only that one time point. Subtractive clicking on any point will remove it from the the set if it is included.
Normal click out of a waypoint unselect all.