Possible defense against a knife held at the small of the back.
Rather than thinking that this is a realistic defense against a knife. I'd suggest to look at it as a two person drill to help understand and develop skill using a progressive and of course practice the actual knife disarm.
Disarms in general take massive amounts of practice to be useful.
Disarms are very opportunistic in their application; IF you find yourself in a situation where a disarm presents itself, that's the time to use it.
Disarms are very, very tricky to apply in real time, without a cooperating partner.
Prof. Presas told me "you will be cut" if you try to defend against a knife.
The reason to train disarms; A last ditch effort to survive the assault.