The sensor detects movement. If you're a 1200 lb draft animal but don't move an inch, you very well may not set it off,... But if you're a 200 lb man jumping up because of a touchdown, you will. I don't know the exact engineering behind it...
By the way,... I can't for the life of me figure out how to adjust the sensitivity (I use a MCV Vera1 running UI2). There's no setting that says "sensitivity," at least not in the Vera's interface,... and nothing like that in the LinuxMCE web admin screens either.
I'm at a loss,... and this is a secondary concern right now. I'm actually more interested in how to turn my Wowwee Rovio into a surveillance camera...