Paladin Tools PA1903 Adaptor Rj11 Modular 6-Way Blister

Does the short cable detach from the device, or do I have to buy a coupler to extend the length?

Will this show if there is a 'live' signal coming from a ethernet outlet in a wall? The router and access panel are elsewhere in the apartment.

what exactly does this device do?

Someone mentioned below that the "line cord should be long enough to loop to other end of adapter." How does that work? Does a signal pass through?