Let's be clear that the video you posted mentions nothing about ES/SD's. . . only that his tuner-break with reduce group size. We're not just talking about a break. When we're talking about ES's and SD's, that usually has to do with velocities. Though a group's ES involves group size. You didn't specify which.
I do use Cortina's tuner-break (like the on in the video pic you posted) on my 6.5 PRC and yes, it has reduced my group sizes. A break alone will change a POI, but they are not a "tuner". Tuning is about getting the timing for the bullet's barrel time in sync with the harmonics of the barrel where the bullet is being released at a time in the vibration that has the least movement. If velocities could be exactly the same, which they never are (except rarely where 2 or 3 in a row are by chance), you can have one small hole when the bullet is released that the very same time in the harmonic vibration. In the pic below, I was playing with the tuner setting starting at 6.5 and moving it down to 4.0. I like the setting from 5.0 to 4.0 and find I can keep the groups touching in that range making small adjustment according to ambient temperature changes. The setting at 6 and above just never gives me the smaller groups . . . having tested this more than once.
Diverting the gases may help in some small way, but that can't be tested unless one uses the same tuner with only the difference in the way the gases are diverted. The gases that Cortina refers to escaping before the bullet, is mostly blowby gasses that start down the barrel before the bullet is fully engraved and is of a low mass having little effect, if any, on the bullet. The tuner itself is the thing that can really produce any change. . . not that gas.
Keep in mind that any time something is attached to the barrel, that alone changes the harmonic vibration and will effect POI. Breaks of different weights, shapes and sizes will effect that differently and in some cases may act a little bit like a tuner making for a somewhat smaller group . . .or even larger. Which break on a particular barrel will do that, no one can say any more that one can say a particular setting on a tuner works for the particular barrel. It's would have to be trial and error test of breaks to see which might do that.
There has been extensive testing by many, many shooters using various tuner's and tuner-breaks and they all work. I've used 4 different ones that all have worked, but some seem easier to use and get a result than others.
Here's that target from my 6.5 PRC I did back in May:
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