Danny Deger wrote:

While in the atmosphere the main engines are throttled back to reduce the
maximum aerodynamic pressure. The grain in the SRB is also designed to
have them go through a period of reduced thrust at about the same time.

Not sure what you mean by "grain in the SRB", but the physical propellant
shape is designed the reduce the thrust by approximately a third 50 seconds
after lift-off to prevent overstressing the vehicle during maximum dynamic
pressure.

Hilton