In His Will and Testament, Abdu'l-Baha provides for the Universal House of Justice to function in two ways: With the Guardian or his representative, or with only its elected membership. Provision for this is not only in the Will and Testament; it is explicit in another Tablet written by Abdu'l-Baha. In both instances, Abdu'l-Baha was in mortal danger, and He arranged for the Universal House of Justice to be elected following His passing. At the time, Shoghi Effendi was a young boy, unable to assume his functions as Guardian of the Cause. Abdu'l-Baha provided for the House of Justice to be established and to function with only its elected members, and provided that it would be divinely guided when it did so.
As the Universal House of Justice has explained:
“At a time described by the Guardian as 'the darkest moments of His [the Master's] life, under 'Abdu'l-Hamid's regime, when He stood ready to be deported to the most inhospitable regions of Northern Africa,' and when even His life was threatened, `Abdu'l-Bahá wrote to Haji Mirza Taqi Afnan, the cousin of the Báb and chief builder of the 'Ishqabad Temple, commanding him to arrange for the election of the Universal House of Justice should the threats against the Master materialize. [The Will and Testament p. 20; The World Order of Baha'u'llah, p. 17] The second part of the Master's Will is also relevant to such a situation and should be studied by the friends.” (Messages From the Universal House of Justice 1963 to 1986, paragraph 23.15, p. 53)
In this Tablet, written during the boyhood of Shoghi Effendi, Abdu'l-Baha directs the relatives of the Bab together with the Hands of the Cause of God to carry out the provisions of His Will and Testament and arrange for the election of the Universal House of Justice.
In like manner in the Second Part of His Will and Testament, written when His life was in imminent peril, He directed that the Universal House of Justice should be established:
“O dearly beloved friends! I am now in very great danger and the hope of even an hour's life is lost to me. I am thus constrained to write these lines for the protection of the Cause of God, the preservation of His Law, the safeguarding of His Word and the safety of His Teachings.” (The Will and Testament, p. 19)
In that same paragraph of His Will He explains that the Universal House of Justice is to be elected; and He promises that the decisions of its members – its elected members – are “the truth and purpose of God Himself.”
“Unto the Most Holy Book every one must turn, and all that is not expressly recorded therein must be referred to the Universal House of Justice. That which this body, whether unanimously or by a majority doth carry, that is verily the truth and the purpose of God Himself. Whoso doth deviate therefrom is verily of them that love discord, hath shown forth malice, and turned away from the Lord of the Covenant. By this House is meant that Universal House of Justice which is to be elected from all countries, that is from those parts in the East and West where the loved ones are to be found, after the manner of the customary elections in Western countries such as those of England.
“It is incumbent upon these members (of the Universal House of Justice) to gather in a certain place and deliberate upon all problems which have caused difference, questions that are obscure and matters that are not expressly recorded in the Book. Whatsoever they decide has the same effect as the Text itself.”
(The Will and Testament of Abdu'l-Baha, pp. 19-20)
Please note that after stating that “By this House is meant that Universal House of Justice which is to be elected,” He directs that “these members”, i.e. the elected members, are to convene and deliberate. He does not, in this Second Part of His Will, written when His death was imminent, mention the Institution of the Guardianship, or Shoghi Effendi, who was then a boy of 11. In this way, Abdu'l-Baha provides for the Universal House of Justice to function as the Head of the Faith, infallibly, without the presence of the Guardian. I have gone into this in more detail here.