Jerry is correct that a 406 MHz ELT or EPIRB sends out a 'ping' every 50 seconds. It is not a continuous signal which means the DF needle snaps to the relative bearing of the signal (let's say the 3 o'clock position) and locks into that position until the next ping is received. During the ensuing 49 seconds, no matter how you maneuver the searching aircraft, the needle will point to the 3 o'clock position (like a dog chasing his tail). This is why the continuous 121.5 signal is important to help us find you.
We launch out towards the gps position provided by your ELT (please register and connect you ELT's to a gps source!) or the derived SARSAT position and DF on the 406 signal until we get close enough to pick-up the 121.5. It is as easy as following the head of the needle at that point. If the needle swings, we went to far.
I can't speak for the CAP, but the USCG helicopters can DF on any VHF-AM (121.5), VHF-FM (marine Ch 16), UHF (243.0) or 406 frequency.
Trivia: there are actually 6 different 406 MHz frequencies used by ELT's and EPIRB's
Flip
U.S. Coast Guard MH-60T pilot