You just explained how?
You
tripped: "To cause to stumble or fall." Logically, because you mentioned your forearm, in this usage of the word, clearly you did not just stumble, but you did actually
fall: "To drop or come down freely under the influence of gravity."
So, logically, if you attempted to take a step, but instead you tripped, which included a fall, your body headed towards the street that you were attempting to cross, under the influence of gravity. Your body, through unconscious reflexes, does attempt to protect itself from serious injury when possible. In this case, as indicated by your mentioning of the forearm, your body could tell that you were about to face plant into the sidewalk. At the time your brain, even if under the influence of strange manufactured chemicals, was able to process the options.
Your brain, in mere microseconds, was able to determine that if it continued on it's normal course, and perhaps your natural state, doing nothing, that there would be damage. It likely ran a few projections and realized that, based on shared experiences, that it would hurt tremendously if it allowed your face to slam directly into the pavement. It knew that your nose would likely smash and cartiledge would break. It realized that there would be a good bit of blood loss and that blood cells would likely be put to work repairing cells for quite some time. It knew there was a possibility of the crash actually shutting the brain down for a period of time, during which some dump truck might come down the street and run over your lifeless body, making a bad situation worse. And it knew that, at least in your case, that no one nearby would do anything, and might instead actually cheer said dump truck along. So your brain decided that it wasn't worth the risk and chose to act.
Now the brain didn't check with you. I realize clearly, based on your post, that your conscious mind had no connection to this survival portion of your brain. That's okay, your brain doesn't mind, it just wants to keep itself alive, despite your attempts to destroy it. In fact, it rules over you, whether you like to admit it or not. Oh sure, you could kill it, but then you would, of course, actually kill yourself as well. Your brain knows this, so it continues to control you and do what it wants, regardless of your thoughts and ideas.
So your brain decided to act before you met the dump truck up close and personal. Again, in less than the time it takes for your snot to drip out of your nose, it considered options. It may have considered trying to go back in time and have you completely not trip, but likely quickly rejected that option because of the time and effort involved in creating a time machine, and the fact that you'd have to survive the dump truck mauling in order to build such a time machine. It likely quickly realized that it was severely limited by your flabby physical body to options at this point in time.
Given the small number of options and the weakness of your current muscles, the brain used the only reasonable option it could find in the limited time it had: it chose to sacrifice your forearm. Sure, it knew that there would be an injury and that you would likely cry, but the brain didn't care. It was going to ensure it's own survival and really didn't even mind if you lost the entire arm. In fact, the brain might have considered trying to hold the arm down on the ground until the massive dump truck showed up so the brain wouldn't have to worry about feeding that arm ever again. So, again, clearly without your knowledge, the brain caused your arm, which included your forarm, to just out in front of you before you completed your most unmajestic fall.
Next,
gravity did it's thing (not caring one bit about you, your body, your brain, or even the huge dump truck, other than perhaps enjoying the soothing massage of the dump truck as it rolled over the pavement because yes, gravity is inside the earth). When your body and arm reached the pavement, your arm was in motion due to the aforementioned gravity. The pavement,
due to it's design has a strength stronger than that of your skin. So when the pasty skin of your forearm made contact with the pavement, the pavement did not move -- but your arm did, pinning your skin between your forearm and the pavement. Since your arm continued to move, your skin managed to tear slightly to compensate for the unforgiving surface of the pavement and the friction created by your falling action. This caused part of the skin to be left in the surface of the pavement, removing it from your forearm.
"
Skinned" is when some skin has been removed. So while you clearly were thinking about some kind of perverted anime while your brain was doing all this work to keep you alive, you still did lose some of your skin on your forearm. Perhaps you were distracted by the sexy garbage men driving the dump truck as you struggled to your feet. After the truck passed, you went on your way, and your brain went back to trying to keep you from killing yourself, but being so manly, you didn't even notice the missing skin until a much later time, which is obviously when you made this post.
I hope that helps you understand how you skinned part of your forearm.
You're welcome.
