A pallet jack [link] is a tool used to lift and move pallets... some of them are purely hand-powered, and this idea is for that type of jack.
The reason this idea is needed, is when moving a very heavy pallet on a surface which isn't perfectly smooth (such as sidewalks with expansion joints, or over
a door sill, etc.), the wheels can get stuck. And because of the weight of the load on the pallet, it can be very difficult for the person moving the pallet, using muscle strength alone, to get it unstuck.
The basic idea is to change the wheels which are steered from unpowered wheels, to wheels powered hydraulically.
To use the device, one simply flips a switch, and the lever which pumps hydraulic fluid which would normally be used to raise up the load, pumps hydraulic fluid which turns the wheels. Due to hydraulic multiplication, the operator can apply more force to the wheels using this method than he could by directly pushing.
When not being used to get the jack out of sticky situations, the wheel motor simply circulates hydraulic fluid back to itself.
As a bonus, it can be used for limited dynamic braking -- press a lever, and instead of reciculating fluid, the hydraulic motor acts as a pump, raising up the load.