It's either a squirt that is somewhat larger than average.
or
It's an aftermarket fuel injection controller.  I have read lots of
anecdotes but no independent studies that demonstrate significant
measurable benefits in milage or emission control.  It can also be
used to goose up power at the obvious cost of fuel economy and
increased emissions.  Tweaking settings and tracking down installation
problems do seem to provide entertainment for a lot of owners if the
blogs are any indication.

Stop SHOUTING.  Google is yer friend:

http://www.megasquirt.info/