The name 50-50 was first used in skateboarding to describe what is now called a "truckstand". This is where the skateboarder stands on one of his or her trucks and balances on the tail of the skateboard. It was a popular trick back when freestyle skateboarding was more well known, but still shows up with pro skaters like Rodney Mullen.
Notice that a 5-0 is half of a 50-50.
To learn to 50-50 Grind, folread these Step-By-Step Instructions on 50-50 Grinding.