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Precisely the following moves gain a 10% flinch chance. Moves that already have a flinch chance in the following list do gain an additional chance to flinch when the user holds King's Rock. The total flinch chance for a move with inherent flinch chance p when the user holds King's Rock and the move appears in the following list is given by p + (1 - p) * 0.1. For example, a Serene Grace Pokemon using Air Slash while holding King's Rock has a 0.6 + (1 - 0.6) * 0.1 = 64% chance to flinch.
While King's Rock is somewhat risky to use because of the low activation rate, it is very useful with moves that hit the opponent more than once, such as Rock Blast. If the move hits 5 times, the flinch rate hovers around 41%, making Pokemon with the Skill Link ability good users of this item. King's Rock may also be used in conjunction of the move Fling, since the target will always Flinch after the move is used, making it a pseudo Fake Out, which is useful for lead Pokemon. In most cases though, King's Rock is a very poor choice to use in any competitive environment.