This is a technique used by race car drivers, runners, cyclists, and swimmers alike.  For all of these sports, speed is the key.  One of the main prohibitors of speed is resistance--air or water.  And, the faster you go, the more the resistance.  Drafting is a key tactic for improving your race results.  The idea and the technique are both relatively simple to grasp and employ, but can make a good difference to those that use it.  Below is the basic concept.

Why.  Drafting is done to reduce the resistance your body is receiving from the water.  The less the resistance, the faster you should be able to go.  Even though lane line technology has hurt the ability to draft in the swimming pool, the benefits can still be gained.  In open water swimming, maximum benefit is still possible--just make sure it is not illegal.  As in any sport, you want to use any advantage available to you.

Drafting is like swimming with the current behind you, or more appropriately, it's like swimming with someone pulling you along.

When to use.  Any time you can!  Maximum benefit is obtained with no lane lines separating swimmers.  Benefits are still derived when separated by only a single lane line, especially in longer races.  Benefits are minimal when two or three lane lines separate swimmers.  But, as long as you are not altering your race or your stroke to do it, go for it.  It cannot hurt.

How.  When swimming with lane lines, get yourself next to the lane line between you and your competitor that is just ahead of you.  Try to get your head by his or her stomach or waist area.  Stick to him or her like glue.  That's it.  If the swimmer moves to the other side of the lane as a defensive move, you will loose some of the pull, however.  If not, you're in!  The theory is that your competitor creates forward moving water which sucks you forward.

In open water, you can get into the same position as above, but the best spot is right smack behind your opponent.  Try not to touch his feet on every stroke but get just about that close, and stick like glue.  He or she will pull you along for as long as you desire (if you want to be off to the side a touch, that's OK too).  And, from that vantage point, the effects are huge!

If the person ahead of you is a good speed fit, you can ride their wake all the way to the end, or use the extra energy you saved to pass them up before the finish!  Your mental skills and strategy are what could mean the difference between the winner's block and the booby prize.  All is fair in love and sports (as long as it's legal)!  It is a beautiful thing!

Conversely, beware of those drafting leaches!  If they tag onto you, do what you can to shake them!