In the context of woofers, that rounded metal piece that sticks out where the dustcap would be is probably more accurately described as an "extended pole piece". It causes the magnetic flux to remain more linear on long excursions; it is a heat-sink for the nearby voice coil exactly where such is needed the most (the forwardmost edge); and it eliminates the possibility of dustcap break-up modes. But I think it is far too small in relation to the wavelengths woofers reproduce to have any significant effect on the radiation pattern, phase, or frequency response.
I do not know why it's called a "phase plug". My guess is that a marketing department rather than an engineering department was the first to use that term for a woofer's extended pole piece.
Imo on a woofer the function of an extended pole piece is very different from the function of a phase plug in a compression driver, which is to give substantially equal-length pathways for the sound travelling from different areas of the concave diaphragm to the exit, such that the recombined wavefront emerges effectively "in phase".
My understanding is that, if it is well designed and the manufacturing tolerances are adhered to, there should be no significant net airflow through the annulus between the extended pole piece and the voice coil at high sound pressure levels. There would be so much flow resistance at low frequencies and high SPLs that the annulus is not in effect an opening. Along similar lines, a port is not a "bass leak" north of the tuning frequency. I once calculated the tuning frequency of the annulus on such a woofer and it was way below the audible range, and that was without taking boundary layer effects into account.
Here is a link to a discontinued Eminence prosound subwoofer which is obviously emphasizing the heat-sink function of its extended pole piece:
www.audioexcellence.ca
