not holding on to anything while riding the subway train, when the train turns, stops, goes etc. you get jerked around and have to "surf" to maintain balance
derived from the natural position your body takes in order to maintain an upright position
can be both purposeful or accidental (e.g. children are often urban surfing for fun, while adults are forced to urban surf when the trains packed)
The other day these two kids almost knocked me over because they were urban surfing from 110th to 96th. Nobody can survive those rickety turns when the train cuts through the park!
The ride to Coney Island was so long that we were urban surfing just to pass the time.