"If you backstab a Razorback-wearing Sniper at about the shoulder, you should be able to get a successful backstab without activating his Razorback."
http://forums.steampowered.com/forums/showthread.php?t=2105464
don't know if correct
http://forums.steampowered.com/forums/showthread.php?t=2105464
don't know if correct