She 'handed the reins' of the company to her successor. = She 'handed over the reins' of the company to her successor.
He has people working for him, but he 'has/keeps a tight rein on' every part of the process.
We need to 'keep a rein on' our spending. [=we need to limit/control our spending]
(..) here comes Santa Claus ♪ ♪ Right down Santa Claus Lane ♪ ♪ Vixen and Blitzen and all his reindeer ♪ ♪ Pulling on the reins... ♪ Yeah, I appreciate the ride... ♪ Bells are ringing, children singing ♪ ♪ All is merry and bright ♪ (..)2014 TvShow: The Big Bang Theory Title: The Clean Room Infiltration Season: 8 Episode 11