A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 19 February 2013
What is the timing constraint on Yoritomo Iwashi's ability? Iwashi's ability states "Limited: If Iwashi has been in an attacking army when it won a battle since your last turn began, target a Personality: Bow him." I've essentially been reading that to mean that it can only be used on the same turn in which he won a battle, meaning only after an Ambush, the battle created by Aramasu's Legacy, or some other method of creating a battle during your own action phase, but now I've been made to question that. So I suppose the question is, are your "last" turn and "latest" turn the same turn, or can I win a battle with Iwashi's army on my turn, go through my opponent's turn, and then, on my next turn, activate his limited ability?
This is the only ruling on Iwashi I could find, but it doesn't deal with this question. http://www.alderac.com/forum/search.php?keywords=iwashi&terms=all&author=&fid%5B%5D=59&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
Jon Palmer (rules official), 19 February 2013
What is the timing constraint on Yoritomo Iwashi's ability? Iwashi's ability states "Limited: If Iwashi has been in an attacking army when it won a battle since your last turn began, target a Personality: Bow him." I've essentially been reading that to mean that it can only be used on the same turn in which he won a battle, meaning only after an Ambush, the battle created by Aramasu's Legacy, or some other method of creating a battle during your own action phase, but now I've been made to question that. So I suppose the question is, are your "last" turn and "latest" turn the same turn, or can I win a battle with Iwashi's army on my turn, go through my opponent's turn, and then, on my next turn, activate his limited ability?
This is the only ruling on Iwashi I could find, but it doesn't deal with this question. http://www.alderac.com/forum/search.php?keywords=iwashi&terms=all&author=&fid%5B%5D=59&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
Your last turn is your turn prior to this one. Iwashi can be used if he was in a winning attacking army during the prior turn's normal attack phase; if he couldn't, he would say "since this turn began".