The rules on voting are quite simple:
Voting on general matters as determined by the Committee, may be put to the Branch meeting and decided by a show of hands. On all issues of a substantial nature voting shall be by means of a postal ballot of the relevant section of the Branch Community directly affected.
Where an issue is deemed by at least a two-thirds majority vote of the Branch Committee to be of such an extreme importance and urgent nature and where a postal vote would be impractical or inappropriate decision could be placed before a BASSA 1/2000 Branch meeting for an immediate decision.
It looks like a simple majority by show of hands. You may find a closer textual analysis informs you differently. I think the second part of this clause is a clumsy addition and it has been pointed out elsewhere that as such it may be unconstitutional because it subverts the original meaning instead of qualifying or moderating it.