What is the test for requiring an accused to explain or contradict?

And later, quoting from Abbott, C.J. in Rex v. Burdett (1820) 4 B & Ald 94, 106 ER 873, said: “‘No person is to be required to explain or contradict, until enough has been proved to warrant a reasonable and just conclusion against him, in the absence of explanation or contradiction; but when such proof has been given, and the nature of the case is such as to admit of explanation or contradiction, if the conclusion to which the proof tends be untrue, and the accused offers no explanation or contradiction; can human reason do otherwise than adopt the conclusion to which the proof tends?’ ”