I’m curious to know what the black part on the rear of the head of the speaker is/does.

It’s a stunning design.

Dedicated theater room: 30'x15'x9
11-4-4 Atmos System. Trinnov Altitude16, 271, DSP750 LCR, DSP7200.2, sides, 7200SE Rear, 5200.2 front sides, 4 x 320.2 atmosphere, 4 x 24" GSG Subs, 2 x Stereo Integrity 12000 (6000w per sub each with own 30amp circuit)
JVC NZ900+DCR Lens Projector, Kaleidescape system.