What the heck is a "bank angle sensor"?

[Tazshido]

I was perusing parts on Ebay and found this;
Triumph Rocket III 3 Touring BANK ANGLE SENSOR:eBay Motors (item 130323451346 end time Oct-06-09 06:20:39 PDT)

So what the hades is a bank angle sensor? Does it shut off the motor if you drop the bike or something? I've never heard of one before. On anything but a rock crawler.

 

[kcc11]

You're right. Is the oh sh!t switch. When the lean angle is to far it will shut the motor off.