There are dozens of expressions which have their origins in horsedrawn days:
Suppose you are in charge of an important job – you are the king pin. (the king pin is the centre of the forecarriage on a 4 wheeled vehicle – the bit that holds front and back together).
You want work done post haste ](mail travelling on horseback before the introduction of mail coaches, and recognised as the fasted form of transport available – also the origins of the post rising trot or posting).
You have to whip up some enthusiasm in your team who are all rather slow coaches. Set the pace and encourage them to put their shoulders to the wheel ](as passengers might have had to do if their coach got stuck in the ruts on a muddy road)
You may be full of beans and raring to go (like a hackney cab horse too well fed and liable to stand on end). However its no use getting hot under the collar because others are slacking (an impatient horse sweats where the harness touches especially under the collar, and a lazy one in a pair lets his mate do the work so his traces hang slack).
You must tell them to buck up their ideas: you might ginger them up (dealers with a horse to show off to customers might encourage him to prance and step out by putting a lump of ginger in his rectum or sheath!!) Of course if they are real old stagers or old sweats (horses who were used in public coach work did one stage out and another back, sometimes both in one day, known as working 2 sweats) they will know their work inside out and have it well in hand (like the driver of a team). Though a handful can get out of hand very quickly.
You must be careful they don’t drop off to sleep (like the passengers sitting on the top of a coach, to whom sleep was a danger!)
You might need to curb (the chain under the chin) your enthusiasm, now matter how headstrong you might be. Once the job is over you can hand over the reins to the next person to take it on to the next stage.
You need to hope that it's not a duffer that takes it on. (the bottom bar on a driving bit is known as the Duffer's bar - if you need to use that then you're an idiot)