At low compressor speeds, i.e. during start and stop, the compressor delivers a
lower compression ratio and that calls for a smaller degree of annular duct
convergence.
That means that at lower speeds, the front stages of the compressor
tend to be stalled and the rear stages tend to be choked. This problem increases
with the number of stages and the pressure ratio, but can be managed by using
bleed-off valves and/or variable guide vanes. Both means are used when needed.
Simplified, the bleed-off valves cut off a part of the front stages by bleeding air
from an intermediate stage, the variable guide vanes decreases the airflow to the
rear stages by throttling the first stage(s).
A twin-spool compressor (each of the two compressors driven by its own turbine)
allows the relationship between the capacity (speed) of the two compressors to alter
without need of variable guide vanes or bleed valves.