As the momentum of a surfcasting rod generates an enormous amount of power, the line would hit the water with great speed and force. The primary purpose of a shock leader would therefore be to cushion the impact on the mainline when the line is cast.
As an example, a size 4 sinker would require a monofilament line of at least 40lb to act as as shock leader to prevent damage to the mainline. This leader should however be ideally at least 60lb, making the entire line less prone to stretching and more resistant to abrasion.
The ideal length of a shock leader for surf casting varies widely depending on the length of rod used. Generally, I would recommend that there should be at least six turns of shock leader in the spool, plus an additional length of leader equal to the amount necessary to run it from the reel to the tip and back to the reel again.
Prior to actual casting, the lead would be usually closer to the upper section of the rod. Therefore the amount of shock leader on the spool increases to 12-18 turns, depending on the diameter of the spool. This method applies to both fixed spool and the multiplier reel.