The video is about adjusting the front derailleur of a performance bike. To adjust the front derailleur you have to first adjust the rear derailleur to the smallest gear. To adjust the front derailleur shift the gear to the largest. If it does not slip into largest gear then adjust the barrel adjuster by turning it counter clockwise till it settles into the largest gear. Turn the barrel adjuster until it stops rubbing against the chain. The full process can be carried out whilst riding, it is simple and can easily be done with a little practice.