Something for beginners to bookmark and learn which also goes for the veterans if they want a refresher course on Airsoft M4 Hop-Up 101 as Airsoft Station does an in-depth video that is almost 23 minutes long... "An in depth look at the hop-up unit of a standard M4 style airsoft rifle. This video covers all parts of the hop up unit, how to adjust it, and how to properly install it in your airsoft gun."