Can someone explain to me what a MAC swap loop test is and how it works?

I work for an ISP providing/managing EVPLs for our customers. We often set up a MAC loop tests to prove our circuit is within the SLA. I know what commands to run to set up the loop for the customer, I know it proves our side of the network is good...I provide the MAC for them.. BUT, not sure what's happening on their end once the loop is set up. I've run into a few situations where I offer a loop test for the customers and they are not familiar with the process. So when they ask what it is and how it works.. Ive had to say.."idk I just give you guys the MAC for the loop, then you run tests" I tried to Google this but wasn't getting a clear answer. Thanks in advance fellow networking folks !