[GAMEPLAY] Ashe - Cheap Shot Interaction
On the tooltip Cheap Shot says it "Activates on damage occurring after the impairment", which correlates to the statement "Note that attacks or abilities that impair the target will not trigger that bonus damage themselves, unless the target is already impaired" on the Wiki ( http://leagueoflegends.wikia.com/wiki/Cheap_Shot ).
Once Ashe has attacked a target at least once in a game, she no longer needs to impair a champions movements to proc Cheap Shot on her auto attacks. Each auto attack essentially has a bonus true damage component to it for her.