Over on www.worldwideaquatics.com/.../glossary.htm
this is their definition (and example) of a negative split:
Neg. Split:
You will swim the second part of the distance
faster than the first one. So a "400 neg split"
means the first 200 time is might be 2', the second 200
is done in 1'45"
Wow. That's almost Ian's World Record.