Can someone explain the lock-on mechanics?

[Jeneki]

So I've got a nice circle around my ship and sucking in all the bullets, cool. Anything that flies into the circle gets a lock-on symbol on it.

... now what? Is there a certain button / combination to press to do something with the lock-ons? I've tried pressing buttons for rapid fire, charge up, and the lock button again, but nothing special happens.

Is there a game manual somewhere?

[pikaboo]

Press D to spawn the circle to lock-on enemies and press same button again shoot laser rockets.