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, 15 May 2010
I did perform a search and did not see anything regarding this interaction.
Will Swift Counterattack stop the "additional" action that is granted by Whispers of the Dying Sun?
Swift Counterattack Open: Give a target Personality and each of his Followers +1F. Reaction: Before the resolution of a Battle action that was taken as an additional action: Its effects do not happen.
Whispers of the Dying Sun Choose a (non-Stronghold) card in play. Each of its base abilities may be used one additional time this turn.
Thanks!
Zen Faulkes (rules official), 15 May 2010
Will Swift Counterattack stop the "additional" action that is granted by Whispers of the Dying Sun?
No.
Additional action is different from an additional use; they both have separate descriptions in the Glossary: http://rules.l5r.com/CE_Glossary_A#Additional_action