There are two (or three)compression seal on the nose of the plug - you can see those in the larger pic above. Additionally, there appears to be a rubber? O-ring at the rear surface of the plug. The plug must be in position to allow the solid extractor to rotate into the vertical position, this also acts as an interlock to prevent closure of the action unless the plug is properly positioned and the extractor aligned to the "in use" position. TCsuggests a 95% reduction in blowback via those seals.

My guess is those compression seals and the overall design of the forward end of that plug dictate the necessity for the removal end (nipple end) having that machined "cut" or reduction at the forward end of the socket. That is likely there to give something to pull on as it seems to me the plug could otherwise be extremely difficult to remove after fouling. It will be interesting to see the removal tool they provide.

As to backing off a breechplug as a general practice, I'm convinced it's a bad one. Just snug, no more and no less, works best for me. In the case of the new Encore's plug, backing it off would only mean it isn't completely in position as those cuts are not really threads.