Begin a numeric argument for the following command.
Digits or minus sign following C-u make up the numeric argument.
C-u following the digits or minus sign ends the argument.
C-u without digits or minus sign provides 4 as argument.
Repeating C-u without digits or minus sign
multiplies the argument by 4 each time.
For some commands, just C-u by itself serves as a flag
which is different in effect from any particular numeric argument.
These commands include C-SPC and M-x start-kbd-macro.