The spring angle is how far crown molding leans away from the wall when it is installed: US standard crown is 52/38 stock, touching the wall 38 degrees off vertical, so its spring angle is 38°. Profiles with a 45° spring are common, and 52°-spring stock exists as well. When the profile is unknown, a framing square identifies it — set the crown in the square and the drop from the ceiling against the run along the wall names the angle.
The spring angle matters because it is half of everything a compound cut needs. Cutting crown laid flat on the saw, both settings are functions of the spring angle S and the corner angle C: miter = atan(sin S ÷ tan(C/2)) and bevel = asin(cos S × cos(C/2)). Feed in 38° of spring and a square 90° corner and out come the famous 31.62° miter and 33.86° bevel that most compound miter saws carry as factory detents. Get the spring angle wrong by choosing the wrong preset and every corner in the room opens up — which is why the framing-square check and a test cut on scrap precede the real stock.