The footbridge rolls on undulating rails cast into the concrete abutments on either bank. Ballast fills the top of each square portal, countering the weight of the bridge deck that connects them. This symmetry allows the whole bridge structure to smoothly role through 180 degrees to a fully inverted position facilitating movement of boats from the river to the dock. This finely balanced is this system allows the 13 tonne bridge to be operated via hand winches only.