A bro's sandwiched arm during the mid-five handshake/pull-in of a bro hug
The bro bar's purpose is to ensure a manly distance between two bros, obstructing any close contact that could be construed as effeminate.
"The only thing that will ever come between you and me, bro,...is our bro bars."
"Dude, I've missed you, too. But, loosen the bear hug a little. You're gonna snap my bro bar if you keep squeezing me so tightly."