It's a preposition ( of ) with a non-finite (gerund) clause complement. ) Some grammarians insist that the subject of such a clause be possessive, thus, of all juries ' being drawn ... The meaning is the same as in its equivalent content clause: that all juries (are to) be drawn from ...
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pokhto develop and enforce the principle of all juries + Modifier (being drawn from)..If you adopt this approach, you end up with an ungrammatical sentence - or at least an incomprehensible sentence. I think your impression that the emphasis is different is due to your thinking of being drawn ... as a modifier - which it isn't. Once you understand