Self defence against knives? No chance.
In the heat of the moment, you've got practically no defence whatsoever. As silly as these videos sometimes are (like the kung fu masters who can magically disarm anyone), if you look at how the big guy suddenly flips and goes mental, the trained student who seconds before had disarmed him can't do a damned thing.
No, if someone comes at you with a weapon you run like the wind and hope you're at least the second slowest person running away.