Funnily enough the shape of the tone-arm. If you have an S-shaped tone arm like that on Technics 1200/10s, then this is supposed to counter the wear you get on the inside of the groove from straight tone arm models. Conversly, straight tone arm models like that on the Vestax PDX2000s are supposed to stay in the groove better when scratching etc. If you ask me there's no real difference and if you have s-shaped tone arms you can always twist the cartridge to get the straight tone arm effect.