To help protect against wind blast experienced through high speed ejection, the visor would automatically lower as lanyards attached to either side of the helmet visor mechanism are retracted into the ejection seat.
These wonderful TSR2 AEA shots show the modified Mk2A helmet with special visor mechanism.
This image shows a modified Mk2A helmet in the centre. The D-Rings can clearly be seen wherere the ejection seat lanyards attach to the rear of the visor mechanism.
Also shown are the modified P2/Q2 oxygen mask to the left, and BWT modified partial pressure helmet, designation unknown, but resembles an E-Type or CX-Type to the right.
I took this photo in 2009 at NAM's Cockpitfest event, way before I built this website so. The helmet belongs to a friend of mine, so one day I hope to re-photograph it properly for this site. However, the modified visor screen and mechanism are clearly seen in this image.