How efficient is iron smelting ?
no. what he was throttling was the liquid fuelled main engines. during ascent, the shuttle experiences a point called "max Q" which is where the aerodynamic forces are at maximum. the air is still dense enough to generate significant forces at that altitude, meanwhile the shuttle continues to accellerate, where the two curves intersect is called max Q. As the shuttle approaches the intersection point in the two curves (air density vs velocity). the engines are throttled back to slow the accelleration to delay the merging of the curves to a point at a higher altitude where the air density is lower to limit the amount of structural stress experienced by the airframe. The shuttle had just passed through the point of maximum airframe stress when the burnthrough reached the cyro-fuel in the tank. that command was irrelavent to the burnthrough as it has already begun much earlier in the launch. and by the way, it was mission control that gave the command for throttle-up, not the commander or pilot, they were merely acknowledging the command.
the launch sequence goes something like this: (Times are very approximate, I'm going from memory, feel free to correct my timing.)
T-02 Main engines ignite gradually throttled up to 100%+
T-00 SRB's ignite, shuttle lifts off pad
T+01/02 shuttle clears tower
T+04 shuttle executes barrel roll to orient antenna for line of sight to ground stations
T+15 or so main engines trottled back to about 80% prior to entering Max-Q
T+25 or so, main engines throttled back up to full power (actually exceeds full rated power by 10-20%) after crossing Max-Q
T+40 or so SRB's just about burned out, they are jettisoned and small steering rockets aim them away from vehicle, they burn for a couple seconds more to ensure they clear the vehicle. they drop to the ocean and are retrieved and refurbished. (cost savings on that used to be about break-even which is the most cost efficient part of the whole system.)
T+60 or so, liquid fuel tank is jettisoned. shuttle continues on using the OMS (Orbital Maneuvering System the two small motors behind the hump-like fairings on either side of the vertical rudder) motors.