Possibly better picture for dipping sonar.
Basically, to do this, the helo stops and hovers a few meters above the water, then lowers the sonar probe, and listens. If they hear something susupicious, then they go active, and try to immediately locate the sub. If they don't hear anything, they winch in the probe, fly over a few hundred meters, and try again (usually also being given steering commands by the ship they're operating from; the ship's sonar, being a lot bigger is also a lot more sensitive, and between the two they can pinpoint a sub pretty quickly). Usually helos in this mission are armed with 2 homing torps.