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: https://parttimeaudiophile.com/wp-content/uploads/2019/05/Munich-2019-he-graham1.jpg