How do I draw line starting at X on line AB which is perpendicular to AB?

1. Create line AB.

2. Create intersecting line CD which intersects AB at X.

3. Problem: How do I draw line starting at X which is perpendicular to AB?

(Note: it is possible to draw a line from any point off the line to meet the line at 90 deg but I cannot see how do do it when the line starts on the given line.)

Second question. I sometimes see the comment Deferred Perpendicular when I hover the cursor over a line.m What does it mean?

Thanks