Jim,
Yes it is secured at both ends, I have a double pulley at the top of my mast 1 for a burgee and the other for the Secumar device. I tested mine when I got it and it goes off when it is about a yerd below the surface, then because it is on a halyard it floats but the mast head is still 6-9inches below so you can control the righting moment, as regards experience mine went off about 60 miles out on the way to Holland a couple of years ago, we had to drop the mast to make a repair and I forgot about the device so it went pop.