Prior to the whole magic thing, which is still odd to Buffy considering there shouldn't be any magic, Buffy is doing the whole stake and bake thing as well. She tends to go 'round back, letting Faith draw the attention of most of the zompires.

By the time the barrier goes up, Buffy is inside, setting the scythe aside and moving toward Angel. This is his show, after all. Buffy is just here to be told what to do. It's not her usual position and she's not incredibly comfortable there, but she's willing to do whatever it takes to save Dawn. This is the best option they've got.