When practicing kyusho, at least the way it is practiced by the folks at Kyusho International, tehy describe 3 level sof "knock out"
when practicing the goal is to use the lightest force that will cause a reaction. When applying, the plan is to use as much force as you have. A lot of people see the light contact PRACTICE and think that KI is teaching their people to use light touch in a fight.
Teh 3 elvel sof "KO" as described in KI practice are:
level 1 - a discernible body reaction such as a weakness in the knees or involuntary eye movements. Difficult to detect on a You Tube video, but obvious in person / on your partner.
level 2 - disorientation, loss of balance "physical/mental disassociation". Could result in falling, or stumbling; dazed, but not completely unconcious
level 3- brief or prolonged unconciousness
So what you are describing above, falling back, could in many cases be level 2. also, some level 3 KOs invovle an involuntary muscle reaction, specifically I am thinking of a stiffening of the legs, that could cause an unconcious person to fall backwards, especially if struck front-to-back. not all KOs invovle rag-doll go-limp reaction (see Rich Franklin vs Nate Quarry)
