Suppose you find yourself off the front in a smoothly rotating small group of two lines, the one moving faster and going to the front, the other slower and drifting to the back. You only hit the front for 5-10 seconds, and then the rider behind you has rotated in front of you.
This is called a double paceline.
It looks like this:
Now think of another time you were off the front. This time you found yourself in a single-file line, with each person taking a pull, then falling off and latching on the back, like this:
Which is more efficient?
The short answer: it depends. It depends on two factors: group balance and size.
Marx used to say, "from each according to his ability, to each according to his need," and while this notion may not necessarily be the basis of thriving economies, it is an element of efficient breakaways. Rotations should allow stronger riders to contribute more and weaker riders to contribute less.
Single pacelines work better when rider ability differs most. At the extreme, the stronger rider simply sits on the front for the entire duration. Double pacelines can only go as fast as the weakest rider can rotate.
When I first started doing goon rides, my favorite turned out to be the 7am. We'd fly up Clara Barton at 30mph in a 25-rider double paceline, and my power output would stay at FTP for all but 5 seconds.
The double paceline works best when there is a large group of well-balanced riders. It limits the variance of power output, since riders need only hit the wind for a few seconds, whereas in single pacelines not only is the lead rider in the wind for the duration of the pull, but also as he or she drifts off the front and tries to accelerate on to the back of the line--surely one of the most agonizing parts of pacelining.
Then there were the times we simply tried to hold Chuck or Russ's wheel. Our double paceline quickly became a single one. We lined out because it was efficient.
Costs and Benefits
Settling on a paceline type--double or single--may seem like a simple choice, but I've been in several breakaways that have failed, possibly because the group made the wrong choice.
A single paceline is more efficient, since it subjects only the front rider to the wind. On aggregate, a single paceline will expend less watts to attain a certain speed than a double paceline. Aerodynamic efficiency is a result of minimizing width and maximizing length--producing the smallest possible frontal surface area.
A double paceline is always more equitable, since it subjects everyone to the same amount of time on the front. The downside of a double paceline is that it is twice is wide, and having two riders on the front will always be less efficient than only having one rider on the front. The load is therefore more than in a single paceline; to its credit, the load in a double paceline less volatile (since the pulls are shorter) and is inevitably fair (since the pulls are are the same length).
Crosswinds tend to make double pacelines more efficient, since the riders on the windward side can shield their teammates from the wind, and increase the overall speed of the group. Being in a single, narrow line, front-to-back, may not be as efficient as wider, oblong echelon.
More often than not, the breaks I've been in--even small, poorly balanced breaks--choose rotating, double pacelines. This is partly a measure to ensure equality. Everyone takes a pull. It also makes the act of pulling shorter and leaves the guy on the front less vulnerable to attacks.
The expectation of riders in MABRA is of a double paceline. This is unfortunate, because the double paceline for groups of less than 10 is slower.
Take a look at TTTs, which are essentially a group of 9 riders trying to go as fast as possible. The concern is not equal distribution of work; it's maximization of work. And there are very few instances of double pacelines in TTTs.
Here's a video of Sky training for a TTT using...a single paceline (*bonus: watch for the flying water bottle and Wiggins' fist shake at 3:20)
From last year's World's TTT UCI Championship:
And this illuminating example from this year's TTT at Tour of Qatar, in which BMC moves to a single paceline in the last kilometers, relying on Taylor Phinney's engine to power them to the win.
The key difference between TTTs and breaks is trust--TTTs have it, breakaways don't. This is probably what you should consider if you find yourself in a break--do you trust your breakaway partners? If you do, you may want to suggest a single paceline.