Why does the MiG 21 use anti surge shutters?
Normally, such shutters are close to the intake and covered by simple spring-operated doors which open when the pressure in the intake duct drops below ambient pressure at low speed.



AV-8B air intake (picture source). The upper two doors are open while the lower three are closed.

The ideal area of a jet intake varies with flight speed. At low speed, it needs to be large so the amount of air swallowed by the engine can be supplied without too much resistance. At high speed, a big intake captures and compresses more air than the engine needs, which increases drag. Especially fighter jets need an intake which is optimized for high speed flight, and in consequence much of the engine power is needed to suck in enough air during the take-off run. Adding doors which are sucked open in that condition will add another pathway for air to reach the engine, which in turn will help to avoid a compressor surge and improves thrust at low speed.

I have no detail knowledge of the MiG-21 intake, but it appears to me that the anti-surge doors are only open at low speed and high engine RPM. At high speed, the pressure in the intake duct is a lot higher than the ambient pressure and the doors are firmly closed.