Bass Guitar Track Used as Sub

So I've been taking the Warren Huart courses on mixing Rock albums and the engineer puts a send on his Bass Guitar track. This send is then used to create a Sub Frequency Bus similar to when engineers use that trick on Kicks. Interestingly the engineer didn't get into great details on that item besides "I send the bass to a sub bus". I've found material and learned how to do it on Kicks but wondering if anyone has used this approach on their Bass Guitar tracks and how did you approach it? Thanks

When he described when adding the bass guitar sub track to all the other bass guitar tracks that's when all the neighbors complain and call the cops. I thought the same when I was writing this and watched it again to make sure. Probably some ancient studio secret :)