Stench has a 10% chance of making the target flinch when hit by a damaging move. Stench does not stack with King's Rock or Razor Fang.

How does stench ability work?

A Pokémon with Stench has a 10% chance of causing the opponent to flinch when hit by a damaging move (provided the ability-bearer strikes first as usual). Stench does not stack with the items King's Rock or Razor Fang, so an ability-bearer holding either item will keep the flinch chance at 10%.

Does King's Rock work with rock slide?

No. King's Rock does not effect moves that has a flinch chance.