The 50-50 is usually the first grind or slide ever learnt and involves sticking both trucks to a ledge, coping or rail and sliding/grinding along. To do this trick approach the grindable object at about a 30 degree angle or less (the smaller the angle the smoother the grind, but it's often easier to approach at a big angle.) Ollie up onto the ledge  and push your back foot out so it lands or the edge (heaps of people don't do that and thus only the front truck gets on.)  As soon as you're on push the board forward (or backward, depending if it's front side or backside) so that the wheels are touching the vertical part of the ledge. (get it?) This makes it heaps easier to balance for longer grinds and you will also grind better this way. Cruise along and as you feel your momentum slow shift your weight to the back kick, drop off and roll away.

Those instructions are for your standard ledge and coping but if you're going for a rail of some sort, either square or round in base DO NOT push the board once you're on as this will end in disaster big time. Also for grinds if you want to them well approach with speed and confidence. This is the most important part of any grind, as if you approach in a slow and reserved manner you'll stuff it and it'll stuff you as well, just remember a grind is A 50-50 IS FAST AND SMOOTH.