The stair climber has three wheels on each side of its base, arranged in a triangular pattern. Some trolleys have a mechanism on the wheels that locks them in a position where only one of the three wheels touches the ground, which makes maneuvering the trolley easier.
The wheels on a stair climber trolley are arranged so that as the user pulls the trolley up stairs, the gap in between two wheels hooks onto the corner of the stairs. One wheel effectively levers on the upper stair and the user applies that leverage to pull up the trolley. The wheels then rotate for the next stair.
Since a stair climber trolley levers a load on stairs, it takes some of the pressure off the person using the trolley. He does not have to completely lift the trolley and its contents, reducing the strain on his body and lowering his chance of injury.