What is the effect of section 27 of the Wills Act, 1837?

That passage was quoted with approval in Re Box's Settlement; Box v. Plant, [1945] 1 All E.R. 547, 172 L.T. 312 (Ch. Div.), another case involving s. 27 of the Wills Act, 1837. There Cohen J. adopted, as well, the statement in Jarman on Wills, 7th ed., p. 785 [at p. 549 All E.R.]: "The effect of sect. 27 is to reverse the old rule, and to throw on those who deny that a general devise or bequest executes a general power the burden of proving by what appears on the face of the will the testator's intention that it shall not do so. This intention must, it seems, be clearly expressed." (my emphasis) in determining whether a contrary intention had been expressed.