I was shooting with autofocus, using a matt box, when I realised that the matt box cut out the autofocus sensor - yet autofocus still worked.

In the book of wisdom (the manual) it refers to the sensor as "External sensor for the instant AF - page 33" but page 33 still left me defeated, can anyone explain exactly what this 'external sensor..." does?

The sensor is for the Instant Auto Focus (IAF). It's meant to supplement the normal TTL autofocus method, allowing for quicker focus on subjects.

The IAF sensor can be turned off, and should be turned off when using accessories that interfere with the sensor.

Thanks Woz.