Disclaimer: I have no idea how the history applies to BMX , but it is easy to keep track of in board sports, where one side of the body is always leading the other.
The origin of the term backside and frontside is an interesting one, and you can say that a trick was done bs or fs quickly and easily if you know the history.
The genealogy (as with most board sports) comes from surfing. When surfing, the wave is the focal point. If you ride up the face of the wave and want to go back down again, you have to turn around. You have two directions that you can turn. You can either turn your front to the wave, or your back. Hence, when turning your back to the wave, that is a backside. Likewise with frontside.
But when you're skateboarding, snowboarding, wakeboarding, or any boarding for that matter, there is no wave. What then? The next step in the evolution: pool skating.
Pool skating was one of the first forms of skateboarding. It was easy to adopt terms from surfing for it too, all you had to do was imagine the sides of the pool were stationary waves. Thus, going up a wall and turning your back to the wall was bs, and so forth. But what if there is no wall, and you just rotate and continue in the same direction? Well, they just simplified a little further.
The next step to simplify it was to imagine whatever side of your body you turn forward first. So if a regular stance skater wants to do a bs 180 ollie, said skater will ollie and rotate 180 degrees clockwise. If the skater is goofy, the direction is opposite. Even more complications arise once you start using rails though.
If you want to do a slide(board, lip, tail, nose, blunt, etc.) on a skateboard, snowboard, or anything else, you have to rotate at least 90 degrees in order to keep moving. You also have to take the rail or ledge into account. The side of your body you turn towards the rail first (or would have turned, had you rotated further) takes the name. For example, a regular stance snowboarder is approaching a rail. The rail is slightly to her left. She ollies and rotates her body 90 degrees counter-clockwise, touches down, and boardslides the rail. She just did a fs boardslide. It is slightly complicated, but once you get it down, you can make fun of your friends for not understanding. And isn't that what boarding is all about?
| 
