User blog comment:Leyrann/Riven Rework - Yes she may be able to use one/@comment-14081808-20150212222635/@comment-4834593-20150214161413

You don't need to use Ki for the "big" ulti actiavtion. You either use the small one at the cost of 140 Ki, granting you an execute ability (but not the classic ult), or you use the big one (which happens when you try to ult when you don't have 140 Ki), and you get the normal ult plus Ki building up and a long donkey cooldown.

And her ult indeed stops Ki loss. And you wouldn't want to wait to build up a lot of Ki, because no matter what you can only buff one ability, while staying back until you have enough Ki will leave you with too little to actually use it even after 15 seconds. The idea is basically: You HAVE to fight before you can use you execute, though you can use your ult with the long cooldown option to have to fight less in order to be able to.