First of all, this is not a movie review. There have been several wonderful film reviews of the great gods in this movie, which have introduced the detailed tactical analysis of the Allied destroyer escort. However, no one seems to explain the HUFF DUFF (High-frequency direction-finding) mentioned in the movie at 09:28. Here's what I found on the wikipedia page to fill the gap:
HUFF DUFF "Huff-Duff" is a nickname for High-frequency direction-finding. A radio direction finder (RDF) introduced by the Allied forces in World War II, often referred to by its abbreviated HF/DF nickname. High frequency (HF) refers to the radio frequency band that can effectively communicate over long distances; for example, between a U-boat and its land headquarters. The Half-Duff was primarily used to capture enemy radio broadcasts, although it was also used to locate friendly aircraft as a navigation aid. HF/DF uses a set of antennas to receive the same signal at slightly different locations or angles, and then uses the slight differences in the signal to show the bearing from the transmitter on an oscilloscope display. Earlier pre-war systems used mechanically rotating antennas (or solenoids), and operators would listen for peaks or zeros in the signal, which would take a significant amount of time to determine, usually on the order of a minute or more. Therefore, the Germans generally believed that as long as they sent a short code, their location would not be revealed. But before 1942, the British army had secretly developed a mature Half-Duff direction finder, which could measure the direction of the u-boat when the u-boat began to transmit the first signal. The equipment was installed in the escort fleet from February 1942. Half-Duff allows operators to determine the direction of a radio signal regardless of whether the content can be read. Since the wolves rely on the u-boat to radio the frigate's location, there is a steady stream of messages to intercept. The escort could then run in the direction of the signal and attack the U-boat, or at least force it to dive into the water (resulting in a loss of contact), possibly preventing an attack on the frigate. What's even more impressive is that when two ships equipped with Huff-Duff accompany the frigate, the intersection of the line formed by the two directions can determine the position of the transmitter, not just the direction.
That's why in the movie the Allies were able to know the location of the two u-boats before using the radar.
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