The truly horrifying thing is that what's swinging like a pendulum is you.
Imagine you're climbing a mountain. No, not a The-Hills-Are-Alive-With-The-Sound-Of-Music type tramping mountain, but a hand-hold to toe-hold affair where you're roped to...well, whatever you can find that might stop you falling hundreds of metres through the freezing air to a bone-crunching death.
Aiguille du Dru, France
Now imagine you're dangling in the air from this rope and that you haven't got a single toe or hand hold within reach. All you can do is swing yourself from side to side like a pendulum until you can reach something and get a grip on it.
That manoeuvre is called a pendule - and I pray to every god who may happen to be listening that I'm never, ever in a position to have to do anything of the sort.
Sunday Rest: pendule. This word comes from the Latin pendulus, from pendēre, to hang down.