This was a British tactical manoeuvre designed for employment against U-boats attacking a convoy (1942/45).
If a surfaced U-boat was sighted shadowing the convoy from a position ahead of it, the warships of the escort force maintained their course and speed in an effort to lull the U-boat’s captain that he had not been spotted. Then, as soon as the U-boat submerged, the nearest warship in the first phase of the tactic turned toward the boat’s last known position and increased speed in the knowledge that its propeller noise was masked by that of the convoy behind it.
In the second phase, the warship approached the U-boat’s last known surfaced position while the circle of uncertainty about the U-boat’s current position was still small as a result of the boat’s low submerged speed. This increased the warship’s chances of securing a firm sonar echo and then of delivering a successful attack.