Rule 39A states: "When a stop signal is at Danger the
stop signal next in rear of it and worked from the same box must not be lowered
for an approaching train until it is close to such signal and has been brought
quite or nearly to a stand."
The Signalman will implement this, aided by his track circuits telling him where a train is, by waiting until an approaching train has come to, or has almost come to, a stand at the signal in question before pulling off the signal and allowing the Driver to take his train onward. The Driver will interpret this as an indication that the following stop signal is at Danger.