...so they can control the bleed on all the microphones.
 

No idea but the vid was worth a watch.
 

It would seem they need to be Gold Plated headphones for best effect! 
Cool Video. Like the music. Is this a client of yours? 

Not a client of mine.
 
I'm just a fan of good performances, and I kind of like the jam in a van concept. I was surfing YouTube this morning and stumbled on the channel and liked what I heard, and I liked how they were hearing what I liked. :-)

Straight out of the '60's. (headphones)
Needs bassdrum - cymbal (s)
 
How many people can you fit into that van? (like a volkswagon from the '60's ?)
(half of California or all of it?) (like the '60's)
 

Often I've recorded 2 or 3 people who are not professionals and are not at all comfortable with headphones. So I just sit them living room style , mike them up and let them perform acoustically the way they are used to playing together. The small amount of leakage is manageable and I'll take a good performance over technical excellence any day.
 
It works fine as long as they are competent enough to not make real bad mistakes that leak into other mikes.  
It also works best if there is no bass involved. Bass leaks into everything.
Notice that Jam had no bass. Bass should be overdubbed. 
 

I notice that you did not notice the guy playing the bass. He's wearing the glasses, and playing the bass. :-)
 
 
I read a post recently where an engineer was reminding those reading that the Everly Brothers never wore headphones in the sessions he tracked.
 
 
 
Playing music live can produce some nice and lively music.

I thought this performance was, how do you say, off the chain spooky cool:
 
 

The amount of post-prod there is staggering. And at least the lead vocal on the first one is clearly mimed. I'd bet on more if not all.
 
Not knocking them for either of those things, but don't be fooled. A jam in a van does not sound like this.