Ehr, point it where you need it.
Say you're tracking a target on radar and want to measure it's speed using the "measure distance travelled in 3 minutes" method. It's gonna be easier to do with the radar focused on a target, so you get constant update on the navmap, instead of having to wait untill the target is sweeped (with the risk of your radarman not picking up the target and thus not updating it's location).
From my experience, you need to point the beam (as represented on the display) a little to the right of your target. If the blip fades, readjust so that it stays bright.
Think you can focus the beam in a direction by clicking on the display.