[onlypassingthru]

>you can use the energy from the swimmer in front of you to 'spring' yourself forward

This bears out in the real world. Much like a peloton in cycling, swimming directly behind another swimmer can be far more energy efficient than swimming by yourself and feel like you are getting pulled along for the ride.

[nradov]

Even swimming slightly offset another swimmer saves significant energy. It also reduces the chance of getting kicked in the face. We do this when possible during triathlon open water swims.