The tool's outside ring allows you to measure a true heading in degrees from North (360). The inside ring gives you the reciprocal.
When you use the hydrophone, you get RELATIVE bearings, that is relative to your heading. The bow being 360.
Let's say you're going east (heading 090) and a target appears on your port side (to the north of you) it will be reported as target bearing 090. But to reach it (if it's not moving) you have turn to a heading of 360.
If you plot an intercept, calculate the distance the target will travel, if you know the course, you measure with the plotter.
Then you draw the intercept course and measure again, it will give you what heading to turn to.
Try it a couple of times on the chart and it will clear the confusion
