This is a modified hidden blade made by Zean on Knex Innovation, so credits go to him. What I did with this modified version though is I made the arm part for it out of Knex not craft foam and duck tape. I just did this to show it off and give people ideas. I must admit though, this is on of the cleanest best functional Knex hidden blade out there. Here's a link to the original instructions if you want to check it out.
http://knexinnovation.net/showthread.php?1086-Zean-s-Hidden-Blade-and-Instructions . How this blade works is, you push the top of the blade back and let go quickly and then the blade extends. You put back in place by pushing it back and locking it. There is a video in the instructions link at the bottom of the page of the instructions.
Also for those who don't know this, this is from the game Assassin's Creed. And I don't think I'll be posting the Knex brace for the hidden blade because it's pretty simple to make and you can probably make from the pics. Although, depending on how big your arm is, you may have to adjust the brace straps to your fitting.
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