A sailing ship's rigging was a favorite target during sea battles, since destroying the opponent's ability to maneuver or get away would put you at obvious advantage.  Therefore, one of first the things tended to after a battle was the repair of broken gear, and the repair of the sheets (lines that adjust the angle at which a sail is set in relation to the wind) and braces (lines passing through blocks and holding up sails).

The main brace was the principal fore-and-aft support of the ship's masts.  The main brace, being one of the heaviest pieces of running rigging in the ship, was probably seldom spliced if damaged, but probably renewed in total instead.  Splicing this line would have been among the most difficult chores aboard ship, and one on which the ship's safety depended.  This operation was one of such rarity and difficulty in the days of sail, as to warrant the issue of an additional amount of rum to the Ship's Company.  For Sailors today, the term "Splice the Mainbrace" is used to congratulate the company of a ship.  In common usage, the phrase refers to an invitation to celebrate.