What do you know about Baby Sensor Mats?

I am doing a little research into baby monitors. I have seen the sensor mat/pad which lies beneath the baby’s matress. It goes off if it does not detect any sound/movement for 20 secs.
Now, a few concerns…
a) does it go off quite often in the night (scaring you half to death) but usually false alarms.
b) how does it sensor movement when baby is laying still? breathing movements? of which it would have to be very sensitive!
c) Do they really work?!

we had one with our daughter. it senses the breath movements. The only time ours went off was when she finally figured out how to wiggle off of it giving me a heart attack lol.

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