While loop inside switch but before the first case statements in C

What is this called and how is it possible?

    switch (state_in->step){
        while (1){
            case step_a:
                // do something and fallthrough

            case step_b:
                // do something and fallthrough

            case step_c:
                // do something and fallthrough

            case step_d:
                // do something and fallthrough
        }
    }

snippet is from cdecode.c of the libb64 project. The state_in->step variable is not touched inside the loop, and code seems to iterate step_a->step_b->step_c->step_d->step_a->....

It seems as if switch acts as a smart goto statement, pointing inside a while loop? I would like to know how this thing works, but google only gives me switch inside a loop in C.