The idea of keeping the weighted legs knee behind its toe is a cardinal rule that I do my best to follow, for the reason you describe and others. That is no different in YCFS than it was in NAWS, both styles have the same rule of knee.
If your knee is forward of the toe you cannot be pushed backward very easily, however a forward pull can easily offset you. As you know if you have committed that knee too far forward, you are easily moved forward.
One learns these little lessons much more easily when ones instructor delights in throwing him to the floor repeatedly. Mine did, I met that floor (wall, whatever was handy actually) too many times to count until I got the idea and learned how to hold myself to best advanatage.
I think those kinds of lessons are best learned during sparring. I noticed my forms improved dramatically after I learned the "why" of them (hitting a solid object repeatedly at speed teaches you this, trust me).
Unfortunately that's a bit of a catch 22. You don't learn to spar (push hands, even) until you learn the forms, but you don't really learn the forms until you learn to spar.
Like everything else in this crazy pass time of ours, it's paradoxical.
Having learned to "lean" quite a bit, I "lean" into just about everything I do, so for me it's easier and seems to make more sense.
When I "lean" forward, I know just how far to do it so that if an opponent thinks he has me and attempts to pull me forward, I LIKE that.
I have found that it's that moment where my opponent THINKS he has me that I often find I have him. They commit themselves to pulling me forward or pushing me backward or whatever, so once I sense this I can yeild and overcome, usually by allowing them to make the pull (for sake of this discussion, we'll stick with the pull), then stepping into it and reversing the energy they expend back onto them in some way.
Don't forget, that's one reason why we have two legs, to step with.
Just because someone has pulled you forward, even using your own energy, does not mean they have defeated you. Sometimes it is exactly where you want or need to go.
If they have "pulled" you, they have given their energy to you. If you have overextended and "given" your energy to them then that's why you channel at all times, to give you that "out" of stepping into your own center again.
I have, on occassion, even "given" myself to my opponent purposely. Yes this is a bit aggressive, I have to admit that, but it can be desirable. If you have "given" your opponent some energy and they have then amplified it and returned it to you, then there is just that much more returned energy to redirect back at them. I have found that, in real life, this will happen, so I have tried to learn how to deal with it effectively.
On occassion, especially against an opponent who is sensitive and sticking to me without giving up his own energy, I have initiated this aggressive method. It can backfire, and has, but often it has lead my opponent to overextend themselves in some way that I can then use against them.
"Yeild to conquer" taken with a grain of salt.
I wouldn't recommend it, especially if you're not sure you're more sensitive than your opponent, but sometimes I get these crazy ideas when I'm sparring and I'll give them a try. Sometimes they work, sometimes I get stomped. But that's what sparring is about, learning what things work, what don't, and why.
That said, I think I bring my knee a bit further forward than most YCF practicioners would consider sane. I tend to ride that line between "too far" and "just right" maybe a little too close. I have spent a great deal of time finding the happy medium, but that medium was for a style that "sat" down quite a bit further into the knee.
What was fun for me was doing the 2nd, 3rd, or 4th generational styles of NAWS after learning the 5th. Those forms are quite a bit lower in their stance than the 5th gen, and are subsequently a bit more "upright" though the center. The "lean" is not too much more prevelant in the 2nd or 3rd gen than in the YCF style I have been learning. The fourth gen starts to utilise it quite a bit and the 5th gen uses it throughout.
However, I was constantly criticised for my overuse of "lean" in those earlier generational styles (you'd think I'd be used to it by now). It's quite a difficult concept to forget, once you get used to it.
The weighted pivots were there though, all the way through.
Anyway (I get sidetracked, what can I say) I'm used to being quite a bit more "down" through the lower portion of my body than is utilised in what I'm learning now. So I think my knee placement is problematic at this point. I "sit" way too deep into the forms and consequently I find my knee is a bit too far forward for my own good.
I have been working on it.
At first, I was "sitting" way too low, so I brought myself up. But that proved very uncomfortable for me. Then, after watching the tape I have of YCF section 1, I noticed that the person on that tape was sitting down much deeper than I was striving for, and so I went a bit lower.
This helped me out quite a bit comfort wise. But that's when I started noticing my knee. It was consistently too far forward for my "posture". I was, I believe, still shooting for that "comfort zone" I was used to previously.
I have worked, diligently, to correct this. However I have not been completely succesful.
I'm not there yet. I need to practice a lot more to "feel" that happy medium.
I will, as you suggest, work on this much more in future. Your "base" is in your legs, if they are not solid and correctly postured, you are in big trouble.