MAC Settings B737-800

[Frosty Hoar]

Have just calculated some take off performance data ex ACE to ARN for a B737 800 crew , and found that changing the MAC input from normal to 25% on the take off performance computer gave a 400kg increase in to the allowable TOW with flaps 1.

I understand that this involves retrimming the aircraft but can anybody explain exactly how this achieves the increase in take off performance...

thanks,FH.:( :(