Sometimes it takes two people to "raise" a really heavy bell - that is, to get it from its "rest" position, mouth downwards, to its "up" position, ready to be pulled down so that it rings. When not in use, bells are generally left mouth downwards for safety reasons. Raising a bell takes quite a lot of work at the best of times - you have to give it a lot of angular momentum, a little at a time. But once it is raised, only one person actually rings it as part of the changes.