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, 25 December 2008
Say I play against a Unicorn, his turn, he declares an attack, assignes none in the Infantry, I assign my only unit to one province and he Cavaleries his unit to another. He then chooses to resolve the battle in the province where my unit is. A simple question, just to make sure: I can play Ring of Water to move my unit to the battlefield where his unit is, can't I? Or do I have to wait till the resolution of the battle in the province with his unit begins? The first variantIt's obviously more preferable, because I'd get the first action, but sometimes I doubt such a move is relevant...
Ring of Water Water After you successfully resolve your fourth or later Battle action from cards with different titles in one battle: you may put this card into play from your hand. Battle: Bow this card or discard it from your hand: Move one of your target units home or to a battlefield with one or more enemy units. If the unit moved, straighten it.
Brook Cunningham (rules official), 25 December 2008
Say I play against a Unicorn, his turn, he declares an attack, assignes none in the Infantry, I assign my only unit to one province and he Cavaleries his unit to another. He then chooses to resolve the battle in the province where my unit is. A simple question, just to make sure: I can play Ring of Water to move my unit to the battlefield where his unit is, can't I? Or do I have to wait till the resolution of the battle in the province with his unit begins? The first variantIt's obviously more preferable, because I'd get the first action, but sometimes I doubt such a move is relevant...
You don't have to wait until the battle with the Cavalry unit; you may move during your Personality's battle.
I'm sorry that doubt plagues you, but don't worry about "relevance". There is no longer any such Rule.
All you need concern yourself with are the Rules of 1) Presence and 2) Location. (see here)
1) Do you have presence at the current battlefield? Yes!
2) You're targeting a unit with Ring of Water's action. Is IT at the current battlefield? Yes!
The action is legal.