The term crash box is usually used to refer to a transmission that has had the syncros removed. Instead of syncros the gears use a dog ring and collar slider mechanism. Basically, it means that to shift on the street it has to be rpm matched. The advantage is that during racing, it's possible to shift really fast because the syncros don't get in the way. You just bang the gears through. Pretty much all hardcore racing manual transmissions use sliders instead of syncros. Liberty's Gears can convert most transmissions (toploader, muncie, etc) to crashboxes, they call it "pro-shifting".