Here, Rise is mimicking the load factor of a horse putting his weight into the breastplate (with work horses this was/can also be a 'collar'), so you can see how the balance subtly alters. It's also a reference to the expression we all use, “putting your shoulder into it". I'm told there are over a hundred colloquial slang expressions we all use every day that come from days when horses were horsepower and part of everyday life.