He's referring to the tapered toroidal ring around the midpoint of the F-1 rocket engines on Saturn V. The F-1's used this exhaust duct to dump the turbopump exhaust into the nozzle skirt extension about halfway down the length of the 'engine bell'. This turbine exhaust was considerably cooler than the exhaust from the combustion chamber of the rocket engine itself, and therefore "insulated" the nozzle extension of the F-1 with a layer of this 'cooler' (meaning like 1200 degrees instead of 3,000 degree) turbine exhaust gas with the high temperature rocket exhaust in the middle.
That's why when you look at the pad shots of Saturn V's lifting off, you'll notice a dull red and black striped 'exhaust' extending out past the ends of the nozzles for a few feet and then being "absorbed" by the brilliant dazzling white-hot exhaust gases of the rocket engine itself. It's kinda weird looking at it, but that's why it happens.
The old Saturn I's and Atlas and Thor simply dumped the turbine exhaust overboard through an exhaust duct(s) sticking out/down to the side of the rocket engines... that's why when you watch those old pad shots from Mercury and the early probe launches and test launches you'll see this sort slow-moving "lazy" orange, dull red, and black looking exhaust 'rolling' off out to the side of the blazing hot, fast, straight rocket exhaust jet from the main combustion chambers... Saturn IB used a single duct all the four center engines dumped into centrally located between all the four inboard engines, and used the 'skirt dump' method on the outer four outboard engines. Early Saturn I's had four ducts dumping sorta straight out sideways below the fins, that's why the early Saturn I's looked like they were just spraying fire everywhere and it looked like the whole butt of the rocket was on fire!
Hope this helps! OL JR