The H-Factor

SHIFT can do some SHIFTy things (sorry, I had to ). I found this on Cad Addict's website:

• SHIFT while drawing a line will lock the
  perpendicular inference. It is like a temporary activation of the ORTHO
  mode. If you press SHIFT, a tooltip with similar to the macro Icon of
  digital cameras appear to show you that ORTHO mode is temporary active.
• When
  using FILLET or CHAMFER, pressing SHIFT while selecting the second
  entity will override the fillet radius or the chamfer distances with a
  value of 0. Very useful if you want to keep certain radius or chamfer
  distance but once in a while you need a sharp corner.
• When Selecting Grips of an object, holding SHIFT will allow us to select multiple grips (deeply explained here).
• When we are selecting objects, pressing SHIFT will allow us to deselect any of them.
• When
  using TRIM or EXTEND, pressing SHIFT will toggle between one command
  and the other (i.e. if you typed TRIM and press SHIFT and select the
  object, it will extend instead of trim).
• For object snap
  tracking, object points are automatically acquired. If you press SHIFT
  you will avoid acquiring points for object snap tracking.
• SHIFT
  + SPACEBAR: cycle through selection peviews If selection preview is
  turned on, you can cycle through the objects by rolling over the object
  on top to highlight it, and pressing and holding SHIFT and then pressing
  SPACEBAR continuously. When the required object is highlighted,
  left-click to select it. If selection preview is turned off, hold down
  SHIFT + SPACEBAR and click to cycle through these objects, one after the
  other, until the one you want is selected. Press ESC to turn off
  cycling.
• SHIFT + RIGHT MOUSE CLICK: Will call the Osnap contextual menu (see more here).
• SHIFT + TAB: When working with Dynamic dimensions, it cycles backward
  through all editable dimensions (TAB only will cycle forward)