The booster is securely clamped to the launch stand. However the starship is only resting on top of the booster - the two parts of the ship are not clamped together.
Launch clamps are released on command of the engine computer - i.e when it detects that sufficient engines are running correctly it commands a clamp release.
Note that Starship Booster's outer engines are actually started using feeds from the launch pad. They need power and pressurized nitrogen from ground equipment to start. The outer engines are not able to restart once off the ground.
So the clamps are required to keep the vehicle in place until all the engines that will start have had a chance to do so. They're also useful if there is an abort during engine start so the vehicle remains secure.
I may be incorrect and they are now using explosive bolts. I've seen those mentioned elsewhere.
However the engines were on the whole time, producing >1G of thrust along the direction of the booster. So that force is going to hold the StarShip and the booster together. But you would expect some lateral force from air friction to push the stack apart at some point.
Launch clamps are released on command of the engine computer - i.e when it detects that sufficient engines are running correctly it commands a clamp release.
Note that Starship Booster's outer engines are actually started using feeds from the launch pad. They need power and pressurized nitrogen from ground equipment to start. The outer engines are not able to restart once off the ground.
So the clamps are required to keep the vehicle in place until all the engines that will start have had a chance to do so. They're also useful if there is an abort during engine start so the vehicle remains secure.