This is a variation on the "One Short" technique. Look for cases where several areas overflow a row, column or zone by one square.
Consider the zone shown below. The 10 squares shaded pink add up to 10+10+21+8 = 49.
Since the nine squares within the zone add up to 45, the tenth square [marked "x"] must be (49-45) = 4!
You can also look for rows or columns that are one over.
