"SPAD" Signal at Clapham Junction

Can anybody here please explain the exact purpose of the signal pictured below at Clapham Junction?

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It is sited a few metres beyond the normal Starter signal at platform 8. (This is the usual platform the up-fast-main trains go through when non-stopping.) When a train passes, the centre light goes to red, it seems in conjunction with the Starter, and the outer two aspects flash red together for about 30 seconds. The signal nameplate suggests it has something to do with SPADDING, but does anybody know what?

And no, it isn't a quiz, genuinely seeking to increase knbowledge. The capture of the ex-508 coach in the centre of a 455 is purely coincidental, by the way!

From wikipedia:

A SPAD indicator[5] is a separate indicator which may be positioned after a main signal where there is a likelihood of a serious collision at a junction if a SPAD (signal passed at danger) occurs at the main signal. SPAD indicators are mounted against a blue backplate or surround. They are normally unlit but following a SPAD they display a steady red light between two flashing red lights arranged vertically. Any driver who sees a SPAD indicator illuminated must stop their train immediately, even if they can see that the signal pertaining to their line is showing a proceed aspect.

Interesting!

The signal was showing such a display every time a train passed, although I suppose the driver of the train would have been gone before he himself actually witnessed it...