What is the implied undertaking of a solicitor in a civil matter?

In Home Office v. Harman, the implied undertaking was described as a safeguard against abuse of what Lord Diplock referred to at page 300 as the “inroad, in the interests of achieving justice, upon the right of the individual to keep his own documents to himself...” In that case the issue was described in the following terms at page 302: So the questions of law in this appeal are: whether it is the duty of the solicitor of one party to civil litigation, who in the course of discovery in that litigation has obtained possession of copies of documents belonging to the other party to the litigation, to refrain from using the advantage enjoyed by virtue of such possession for some collateral or ulterior purpose of his own not reasonably necessary for the proper conduct of the action on his client’s behalf; and if so, whether a breach of that duty constitutes a contempt of court.

This principle led to the expression of the rule at page 304 of the judgment in Home Office v. Harman in the following terms: …an order for production of documents to a solicitor on behalf of a party to civil litigation is made upon the implied undertaking given by the solicitor personally to the court (of which he is an officer) that he himself will not use or allow the documents or copies of them to be used for any collateral or ulterior purpose of his own, his client or anyone else; and any breach of that implied undertaking is a contempt of court by the solicitor himself. Save as respects the gravity of the contempt no distinction is to be drawn between those documents which have and those which have not been admitted in evidence; to make use for some collateral or ulterior purpose of the special advantage obtained by having possession of copies of any of an adverse party’s documents obtained upon discovery is, in my view, a contempt of court.