I *think* the pads of them go under the sheet and they will sound an alarm if the breathing isnt felt for 20 seconds, and again after 30 seconds I believe.
Someone who has one should be able to explain it better lol.

There is no real need to swaddle, or tuck in tightly etc. If he is on his tummy, this is only going to make it dangerous.
If he sleeps better free, leave him be. The only think swaddling really does is help relive their startle reflex from waking them up. We never had this problem with Charlotte much at all, so again, swaddling wasn't necessary.