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.
Red Leaf Cut Reaction: Before Focus Effects resolve: Discard a focused card; its owner may focus an additional card from his hand or focus pool.
If my opponent ends up focusing a card that is owned by me (via, e.g., Doji Nagori exp or Bayushi Shoju inexp), and I use Red Leaf Cut to discard that card, do I get the additional focus?
Red Leaf Cut Reaction: Before Focus Effects resolve: Discard a focused card; its owner may focus an additional card from his hand or focus pool.
If my opponent ends up focusing a card that is owned by me (via, e.g., Doji Nagori exp or Bayushi Shoju inexp), and I use Red Leaf Cut to discard that card, do I get the additional focus?
Assuming you've actually got a participant in the duel (might not have one in a multi-player game), sure, you can do exactly what the card tells you to do.
You do, however, have to cackle maniacally while doing so or it won't be anywhere near as demoralizing as it is supposed to be.