This device will let you know when an underwater conibear is fired. Instead of having to physically check the trap you can just look for the flag and save time. When the trap is fired it drops down past the first eyelet. In deeper water sets the flag will be submerged if the trap has fired, letting you know you need to check the trap.
This is also a great way to do underwater beaver sets as well. The same principal applies. Weld 2 eyelets on the conibear support then use a surveyor flag threaded through the eyelets and attached to the conibear. This will not slow the trap down enough to worry about it.