[rdl]

How does it work? I assume it's a water pressure sensor which requires more than a foot or two of water to activate, and some kind of timer, combined with the transmitter? (otherwise a collar doesn't really work -- I can keep my neck immersed in water all day and not drown...)

[pcrh]

From another site [1], it seems as if it monitors the amount of time spent underwater, and is activated if this exceeds a preset amount of time.

[1] http://www.slate.com/blogs/future_tense/2013/05/28/seal_syst...