This section is from the book "Real Property, An Introductory Explanation Of The Law Relating To Land", by Alfred F Topham. Also available from Amazon: The New Law Of Property.
1. General Powers. - The object of a general power is to enable land to be freely sold or other-
(e) See more fully Goodeve, pp. 276-278.
(f) Conveyancing Act, 1881, s. 52.
(g) Re Eyre, 49 L. T. N. S. 259  W. N. 153.
wise disposed of. Therefore the land is not tied up by the creation of a general power.
Hence the period of the rule against perpetuities begins to run from the date of the exercise of the power.
Rous v. Jackson (1885), 29 Ch. D. 521.
In 1800 property was settled on the marriage of J and C on trust for J and C for their lives, and after their deaths, if there should be no children, for such persons as C should appoint by her will. C died in 1838, having appointed the property to R (who was born after 1800) for life with remainder to his son in tail.
1800. Creation of power.
1838. Exercise of power.
Thus, if the period ran from 1800, R would not be a "life in being," and the gift to his son would be void.
But Held, that the period ran from 1838, when the power was executed, and at that date R was a "life in being," and the gift to his son was therefore valid.
2. Special Powers. - The object of a special power is to restrict the persons to whom the land may be conveyed. Therefore the land is "tied up " so soon as the power is created.
Hence the period begins to run from the date of creation of the power.
Thus, suppose that in 1800 property had been settled on trust for J and C for their lives and after their deaths, for such of the nephews or great nephews of C as she should appoint. And C in 1838 appointed by will to her nephew R (who was a bachelor and was born after 1800) for life, with remainder to his first son who should reach 21. The gift to the son of R would have been void - for the period would run from 1800 when R was not a "life in being," and therefore the son of R need not necessarily reach 21 within a life in being and 21 years thereafter.
In applying the rule against perpetuities to powers of appointment, the facts to which the period is to be applied are ascertained when the power is exercised (h), with the following result: -
(1) If in the events which have then happened the interests appointed must vest within the life of a person who was living when the power was created, or within 21 years afterwards, the appointment is good.
(2) Otherwise the appointment, so far as it will not vest within those limits, is bad.
Re Thomson,  2 Oh. 199. The testator, who died in 1872, gave property to his wife for life, and after her death on trust for C T and his issue in such manner as the wife should by will appoint. The wife died in 1893, having by will appointed the property to C T for life, with remainder to such of his children as should attain 25, etc. All the children of C T attained 25 during the life of the wife, i.e. before 1893.
1872. Creation of power.
Exercise of power
Held, As the interests of these children had all vested during the life of the wife, who was living in 1872, the appointment to them was good.
If, however, any child of C T had been born after the death of the wife the appointment to such child would have been bad.