To pull a "voller" is to realise that you are ultimately screwed for an exam, and walk out in the first 5 minutes claiming you have experienced a panic attack.

Upon exit from the examination room, you file for a special examination, (dodgy medical certificate needed) due to the above-mentioned panic attack.

You then end up with a HD after sitting this special examination 1 month later.

Best used as a all or nothing, last resort tatic when you are screwed for an exam. Hence, 'pull a voller'