Its an oh crap move if and only if you happen to be in possession of a Bowie knife, which means it's by no means a unfortunate incident you had no prior warning of, .
There's only two viable defences to a knife attack, which are a) to get out of range and b) to get them first, both of which are depend upon xckusivly on speed and anyone slashing at you ( in range ) is leaving themsekfs wide open to a streaight plunge which is why slash attacks are not popular against someone with a big knife. . The general target for a slash attack is the out stretch hand that holding your knife, but then not big slashes that leave you open,
All if this is easy to prove to yourself with a bit of sparring and plastic knives, remembering, that it's a one touch win, or there no win if you are both dead