I was taught to flex further the supporting knee while back moving leg passing . Surprisingly. my book said oppositely, it said : " Delay the back foot for as long as the music will allow, bring the back foot sharply forward to its forward position , the knee flexing more on passing the supporting foot , but taking care not to flex further the supporitng knee."
I have no idea whether the advanced dancer flexes further his/ her supporting knee to draw the back leg forward.

Flexing the back knee more as the moving knee passes it sounds reasonble. In tango, there is a strong trend of the knees towards each other. You could think of it as the moving knee going through, which stretches the "rubber band" and starts pulling the standing knee forward, which in turn makes the body move.

I like the thought of rubber band . I have a sense of acceleration while my body is cruising levelly on slow, q in progressive chasse ( tango). I paid more attention at the moment of straight knee onto my moving heel which I negled before.

Yichin 1. Is there any change in the forward walk Tango. There most certainly is.