User blog comment:Leyrann/Riven Rework - Yes she may be able to use one/@comment-24947613-20150127063155/@comment-4834593-20150127064418

That's indeed one of the two cases, but the idea is that using her ult in the current way is more of a desperation use where you need the Ki and you need it NOW (or, well, in the upcoming seconds) while the execute moves in general are made more accessible but also more telegraphed and shorter ranged. If you flash out, then Riven can't just hit you anyways, she has to use the buffed ability to close the gap instead, which will cost her a lot of potential damage.