Off the top of my head as haven't got my books at hand:
Determin your target. Units within 6" radius might be hit on a 6 (take the test for each units). If they are hit, then follow the normal explosion procedure, placing the template where you can hit the most enemies from the unit.
In any case if for whatever reason models from another unit that's not been properly hit as you failed on your roll of 6, ends up under the template, they'll be hit as normal.
Long story short: rocket launchers are actually mostly a barrage weapon that hits units in a radius.