Chamfer is an interactive tool. This means you control the tool directly with the mouse; select the tool then drag left or right within the viewport. Chamfer converts each selected point to two points with soft interpolation between them. Using this you can, for example, round the corners of a square by chamfering the points once or even several times.
Only the selected points are chamfered. If no points are selected, all connected points of a spline are chamfered. For an open spline, the start and end points will not be included in the chamfer. If a point possesses tangents then these will be set to zero before chamfering.