IIRC its to reduce the shock of going supersonic on the stack. They also throttle back for a short time during the launch for the same reason.

edit: a misssspelled word can change you're whole meaning....

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"Light thinks it travels faster than anything but it is wrong. No matter how fast light travels it finds the darkness has always got there first, and is waiting for it." — Terry Pratchett, Reaper Man

441!!!! :)