In String It Out - sometimes called simply "String" - all players are issued a length of string prior to the round's start. The string can be used at any time during the round by a player who wants to improve his lie.
Example: Your ball is sitting down in heavy rough, but if you could move it just one foot, you'd have a great lie. OK, take out your length of string, string it out in the direction you want to move the ball. If the string reaches the point where you'd like to place your ball, you can move the ball.
The catch is that each time you use your string, you must cut off the amount of string corresponding to the distance your ball was moved. Move your ball 15 inches to get out of a bunker, cut 15 inches off your string (a ball can be moved out of any hazard other than a water hazard).
The string can be used at any time, but, obviously, must be used judiciously or it runs out pretty quickly.
There are a couple options for the length of the string. All players can be issued the same length (usually a yard). In that case, use full handicaps. Or a player can be issued a length corresponding to his handicap. A 15 handicapper would get 15 feet of string; a 2 handicapper two feet. In that case, use partial (usually 3/4) handicaps.