What is a rain cape? It is a water-repellent poncho of sorts. They are supposed to go over the
handlebars and seat, providing a big waterproof tent with lots of ventilation beneath it. To best use a rain cape, you also should use a full set of fenders and booties too to minimize rainwater's assault on your personal comfort.

There are elastic loops built into the front of the rain cape that go over your wrists, and in the back there's a waist loop. The internal straps are there to keep the rain cape in place in windy conditions.

From the Carradice website:

"Bright fluorescent yellow ensures maximum visibility in all conditions
Additional reflectors to front and rear
Hide-away waterproof zip with reflective covering.
Hideaway hood with reflector on the back.
Generous cut ensures excellent ventilation
Taped seams.
One size to fit all, Internal tapes to secure to bike and cyclist, Scotchlite™
front 100cm, back 94cm

Pro-Route Rainwear - this is a range of lightweight rainwear made from
bright fluorescent yellow (to EN471 standard) polyester water resistant
coating
All items are fitted with ScotchliteTM reflective tape for added night safety"