User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-13471981-20140910163215/@comment-3225933-20140911214113

Well, I saw that too. I think, when you are standing behind someone, the weapon icon changes to Hidden Blade automatically indicating that you can assassinate someone. Be it with the blade or bare handed.