The concave ones allow a more full burn -- starting with the pellet nearest the breech. The empty space allows ignition to reach the entire bottom of pellets where the flat plug creates an inside-out burn -- hence the hole in the middle of the pellet. So in essence, the concave plug creates a hotter, more even burn with pellets. That is also why I get more plug seizures with concave plugs than flat plugs using loose powder.

Loose powder becomes more packed against bore walls than pellets -- which are sometimes bore diameter-undersized. As a result of being right against the bore wall, more ignition back-pressure occurs... hence more power/energy there. Kinda why all the powder & bullet manufacturers allow 150 grains with pellets -- but not with loose powder. I bought the concave Omega plug almost two years ago & only used it for my theory testing one weekend at the range. Ever since then, I've opted to use the flat plug.