Ah yes- I read about this a while ago. It seems to be guys who wander around the city looking for cool walls to do flips and jumps off of and backflips and whatever else. Basically just like most any young boy goes outside looking for trees to climb and walls to walk on top of and balance on.
And yeah- it does sound pretty cool and fun. At least it's a healthy use of energy.
Of course, there's a certain similarity with skateboarding and biking here, ala street skating. Though vert is certainly fun too. It's all good at the X Games.
