A virtual sharp creates a sketch point at the virtual intersection point of two sketch entities. Dimensions & relations can then be added to this sketch point & are retained even if the actual intersection itself were to be removed. (such as when a corner is removed by a fillet or chamfer)
To create a virtual sharp in an open sketch, hold down "Ctrl" & select two sketch entities (as shown below). Click Point (*) (in the Sketch toolbar) or click Tools, Sketch Entities, Point. A virtual sharp appears at the point where the sketch entities would intersect.