I found at
http://www.thefreedictionary.com/arch these definitions of the verb "to arch":
"3. To bend backward: The dancers alternately arched and hunched their backs."
If standing, "to hunch" means to bend forward at the waist, bringing one's back convex upward, no argument there. So, "to arch", to be "alternate" to "to hunch", must mean to bend one's back convex downward.
and
"6. to form or cause to form an arch or a curve resembling that of an arch: the cat arched its back."
Put a dog and a cat together and the cat's back will go convex upward.
To use precise anatomical nomenclature, the dancers are doing vertebral extensions when they're arching their backs; the cat is doing a vertebral flexion when it's arching its back: exactly opposite directions!
Is it generally known that "to arch" has two opposite meanings, and people just "know" which meaning is intended? I think this kind of word is called a contronym, but I couldn't find it in any lists of contronyms.
Alan