mouse-drag-secondary

mouse-drag-secondary is an interactive compiled Lisp function in `mouse.el'.

It is bound to <M-down-mouse-1>.

(mouse-drag-secondary START-EVENT)

Set the secondary selection to the text that the mouse is dragged over.
Highlight the drag area as you move the mouse.
This must be bound to a button-down mouse event.
The function returns a non-nil value if it creates a secondary selection.