The 'Perp' keyboard snap allows a quick and easy way select a 'Perpendicular' point on an existing entity. This snap is only valid therefore if a current point exists i.e. selecting the second point when drawing a line.
To use this feature simply position the cursor over an entity e.g. a line, and hit the key. Draft it now displays the snap symbol at the perpendicular position on the entity. The dynamic line previously attached to the cursor also attaches to this point. If you are happy with the point selected click anywhere in the drawing area to accept.
Use of these keyboard snaps also automatically switches to the Snap input mode.