Clicking this button will toggle on or off a feature that allows you to use edit pointers
to indicate what part of the song will be affected by your editing actions.
You could choose to work on only what's between the pointers, or what's outside of the
pointers by clicking the direction selection indicator once you have enabled pointers.
|<-->| In this mode, the work will be done between the pointers.
<-||-> In this mode, the work will be done outside the pointers.