To activate, you must hit L+R at the same time, then fire the beam as regular. You don't have to charge it. To deactivate, hit Up on the D-Pad. I warn you, however, that the beam will sometimes malfunction and cease to work for a while, even if you press L+R and then fire. Don't worry about this, just give it some time, or, if you want to speed up the process, try locking on to something.
REGION: Unspecified | CLASS: Miscellaneous Codes