According to the Job Description for the Exam Technical Delegate the primary purpose listed is:

1. To ensure the facilities and the standards of the examinations are appropriate.

2. To Mediate between examiners, Candidates, parents and coaches.

There are then 10 key tasks listed that the TD is expected to adhere to.

The TD is the advocate for "all" parties concerned (Candidates and Examiners) to ensure a fair and honest exam is conducted.
In broad strokes :
The two Examiners must jointly make the decision to pass or fail a candidate and
the TD can only interject in the case of a failure if the syllabus hasn't been adhered to by the examiner or some other extreme action by the examiner has been the basis of the failure.
If the Examiners cannot jointly agree on a pass or a fail then they might refer to the TD for an opinion on some points. 

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