Anyone interested in working on Knife fighting methods really needs to invest in training knife.I usually carry a Spyderco Delica & Spyderco makes a training version of it that is nearly identical to my carry knife. That way I can work on various draws and movements without fear of accidentally taking off a finger.It also allows you to work with a partner & make a little contact although for any free style work, I'll sometimes substitute a rubber form trainer.
One scenario that exists is the real world defense is the possibility that you might have to cut your way out against a unarmed assailant.If you have two equal sized Males,them this will be usually looked at as excessive for force & illegal use of a weapon,but in some circumstances it may be justified.A woman against an attacker twice Her size, an elderly person being assaulted by a young attacker or being attacked by multiple unarmed attackers.Any of these situations may be life threatening & warrant strong methods to deal with the attack. Even if you are at least "equal" in appearance to your attacker,perhaps he has you in a choke hold you have been unable to get out of & are quickly blacking out. In this case, I wouldn't hesitate to draw my carry knife and draw it across his forearm flexors.The same goes applies if your are mounted by your attacker(possibly hurt) and He is raining blows down on your & you are on the verge of unconsciousness. Not an ideal situation or response by any means ,but sometimes things simply just don't go our way & IMO I'd take my chances being charged with some improper use of weapon charge vs being unconscious & at the mercy of some thug on the street.
The trainer that's identical to your carry knife is a great tool when working on these scenarios in training as it will give you a good simulation of opportunities and method of drawing & applying your knife as a last resort if worst comes to work.