Alleycat races are sort of like scavenger hunts ... or simulated bicycle messenger days.
Participants are each given a package (or letter) to deliver to their next stop, and an address for their next stop. They cycle as fast as they can, using whatever routes they like to that stop.
When they get there, they drop off their package, and pick up the next one and next address. Then they cycle as fast as they can to the next stop.
And so on, until they arrive back at the start having successfully delivered each package.
The fastest one to do this ... wins.
(There might be slight variations on this)
Bicycle messengers often participate in these as a way to prove that they are the best among the messengers because they are fast cyclists who know the best shortcuts.