A spring line is a docking line that leads from the bow or stern of your boat to a point on a pier. This will keep wind blowing parallel to the dock from pushing your craft away from its secured position while it is docked.
A craft fastened to a pontoon is rigged with four lines. Two run from bow and stern cleats and are tied to the pontoon, one runs from a centre cleat astern of the craft (called a spring line) and is also tied to the pontoon.
If you are going to back into a tight slip, tie a doubled amidships spring line so it starts and ends at your boat. Loop the line around a cleat or piling and allow some movement, but make sure you leave enough room for your bow to move side to side.
When you want to spring your boat clear of a dock, rig a clean spring line from the aft cleat on the boat, loop it around a cleat or bollard roughly in line with the midships position, and run it back on board. With the helm centered and engaged in reverse, apply power gently as the tension comes up on the line. Once the stern is clear, release the bitter end of the line, pull it through a block, and you’re on your way.