Both Uses

rein
in
Fences, by August Wilson
(Auto-generated)

I picked up them reins and commenced to whupping on him.
Chpt 1.4 *

reins = straps used to control a horse
I picked up them same reins that he had used on me.†
Chpt 1.4

Definitions:

(1) (rein as in: to rein in their activities)
to restrain or control; or a means of control
The meaning of rein depends upon its context. For example:
- "keep a tight rein on the new employee," or "rein in a horse" -- to control or restrain
- "give the new employee free rein," or "give the horse full rein" -- do not restrain
- "the reins of government" -- means of control
- "the reins of the horse" -- leather straps used with a bit to control a horse (You might like to think of other senses of rein as being figurative derivations from this sense.)
(2) (meaning too rare to warrant focus)
to "give free rein" is not to restrain (not to exercise control over)