How does adding braces to a variable prevent it from being borrowed (owned?) by a match?

Question

I'm trying to understand an implementation of a binary tree in Rust and I can't wrap my head around some scoping magic in the locate_mut function:

match {anchor} {
    &mut Some(ref mut node) if key != node.value.key() => {
        anchor = if key < node.value.key() {
            &mut node.left
        } else {
            &mut node.right
        }
    },
    other @ &mut Some(_) |
    other @ &mut None => return other
}