This command adds a crescendo graphic to selected notes. This is sometimes referred to as a hairpin crescendo. The crescendo will appear on the staff in the same vertical position as the most recent prior Dynamic or Dynamic Variance (which ever appears first).
This first note after a Dynamic change that has this property turned on will trigger a gradual volume/velocity dynamic swell or fade. The gradual change is based on the Dynamic instructions that immediate precede and immediately follow the notes within the hairpin. For longer swell and fade sequences, it is generally preferrable to use the equivalent text instruction using the Dynamic Variance item. You can safely use both items in the same swell or fade, since only the first one that is encountered will actually be used during play back.