Originally Posted by
Uplinker
To do this, they took the shuttle main engines down to 67% thrust in the lower atmosphere and then back up to 104% when the atmospheric density reduced sufficiently.
The SRBs also 'throttled down' at that point by a change in the shape of the propellant at that stage of the burn.
Have a look at this superb footage of a launch. The Shuttle main engines start first - to make sure they are all healthy - before the boosters are started. You can see (and hear) the whole STS angle over with the force, and then the boosters are lit.
Pedantically, the boosters were lit when the shuttle returned to the vertical after the SSME ignition pushed the stack forward and it swung back.