carpediemracing 

I didn't see it either so I googled it.

First you go here:
http://labs.strava.com/flyby/

Then you put in the URL for the Strava Activity you want to view. For me, my example was:
Bike Ride Profile | 4/19/2014 Mystic Velo Crit, melted in the wind. near Charlestown | Times and Records | Strava

Finally you check off who you want to view in the playback from the list of riders. The green C is Correlation - the higher the number the more probable they actually rode with you. The purple S means they rode the same roads but not necessarily at the same time, so in a crit they probably did a different race if their C is low. The red D is how far they rode, if their number is really wacky then either they rode a ton or they got shelled.

I selected everyone that was over 50% for C. You can pause and select more or deselect the ones that are obviously not racing or whatever. For me there was a guy Don at 70% C but he was watching the race from the side so I deselected him.

Finally hit the play button. It's fun. Think of significant events in the race - breaks, chases, primes, final sprint, crashes, and you'll see them sort of played out. In my event there was a slow-mo crash just before the bell and you can see how the field just explodes after that, due to some people slowing hard, a few Strava riders falling, and the field itself just splintering under pressure. It really strings out for the finish.