If the Copper is used to start up a sequence of blitter operations, it
must wait for the blitter-finished interrupt before starting another
blitter operation. Changing blitter registers while the blitter is
operating causes unpredictable results. For just this purpose, the  WAIT 
instruction includes an additional control bit, called BFD (for blitter
finished disable). Normally, this bit is a 1 and only the beam counter
comparisons control the  WAIT .

When the BFD bit is a 0, the logic of the Copper  WAIT  instruction is
modified. The Copper will  WAIT  until the beam counter comparison is true
and the blitter has finished. The blitter has finished when the
blitter-finished flag is set. This bit should be unset with caution. It
could possibly prevent some screen displays or prevent objects from being
displayed correctly.

For more information about using the blitter, see
 Chapter 6, Blitter Hardware .