I'm still basically just learning Beng Bu but I'll take a stab at this.
It seems as if you are relatively close. Not hip to hip close but like knee to knee close with the other end being ankle to ankle close. I think you DO want them to go over the knee because if you are ankle to ankle in this move then you are over-extended as you try to chop the waist and could get countered more easily by being pulled forward at that point.
also, given that TO ME, it seems as if the hook stance is not used dynamically in this application. I feel like you are sneaking the hook in to act as a post to trip them over with the pull of the arm and the chop to the waist. SO, in that instance, it would be easier for them to just empty step out and back if you are just ankle to ankle.
just my novice thoughts....
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