All votes (except membership related votes; see above) require a simple 50% majority. Any member of the QA team may call for a vote at any time. All voting is done on the QA mailinglist.
The core team may overrule any decision (this includes votes on memberships) of the QA team and may decide to hold their own internal vote to make the ultimate decision on how to proceed. Furthermore, the core QA team may also hold votes without even consulting the QA team. However this should be only done in rare cases. All members of the core QA team may make decision on their own if a decision is time critical (like pulling a broken release). However the core QA team should be aware that making too many decisions on their own may lead to alienating the other members and therefore if possible it should always be attempted to at least consult other members of the team.