These arise where the trustee on the face of the will takes as trustee but the terms of the trust are not specified. For example, if property is given to a person for purposes which I have communicated to him or for purposes which he is aware of a half secret trust will arise. It is clearly established that evidence cannot be adduced to contradict the express terms of the will, therefore if the will points to a future communication, for example, to my trustees for purposes which I will communicate to them evidence cannot be admitted of communication made before the will was made. Similarly if the will points to a contemporaneous or past communication evidence cannot be admitted of communication made after the execution of the will. You should note that as the present state of the law stands future communications with respect to half secret trusts, whether or not the will points to them, are not in any even admissible.
However where the communication of the trust is made before or at the same time as the execution of the will evidence is admissible to show the terms of the trust and the trustee is bound by it. You may refer to the case of Backwell v Blackwell (1929) A 318 for that proposition.
It has been argued that the principle governing communication made after the execution of will yet prior to the testator’s death with respect to half secret trust cannot be justified and it would appear that the courts have confused the doctrine of secret trust with the probate doctrine of incorporation by reference. Section 12 sets out the probate doctrine of incorporation. It means that it is possible to incorporate in a will a document which is not executed in accordance with —- in our case-- the Law of the Succession Act but for the doctrine to apply the document must be in existence at the date of the will and must be specifically referred to in the will. It is argued that this rule of probate is concerned purely with the validity of the will itself and the documents to be incorporated within it. The rule should not and does not relate to secret acts, which according to the doctrine of secret trusts operates outside the will.
If a testator wishes to carry out his purpose by making a number of secret trusts piecemeal he must inform the trustees in respect of every edition to the secret objects. Example in the case of Re Colin Cooper (1939) Ch 811 in which a testator by will bequeathed 5,000 pounds sterling to two trustees “upon trusts already communicated to them”. He had in fact communicated the nature of the trusts to the trustees by a farther will he purported to increase the sum to be devoted to the secret trust to 10,000 pounds but did not inform the trustees. The result was that though the first install of 5,000 pounds could be devoted to the secret trust the second installment could not.