the BBC slot... revisited

Can somebody technical explain the advantages of this configuration?

My interest is piqued because Graham Audio will soon be introducing a significantly updated version of the LS 5/5, with just such an aperture (or two) incorporated in the design.

Prototype (not the finished article) pictured here:

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Apparently acts as a low pass filter for the woofer. Didn't get any further but the whole white paper is available for download here: