I would also love to discuss various understandings of zhan, nian, lian, sui. I agree with others that this is a significant characteristic of TJQ, and it is much less discussed than the eight energies (peng, lu, ji, an, cai, lie, zhou, kao), for example. I also understand these to be important for “yielding” and “leading” (including “leading into emptiness” which is the topic of this thread).
I only have limited instruction concerning these terms, so I am not particularly confident in my own understanding, and I found the posts by Doc and Steve to be interesting. Since there are many independent schools of TJQ (not to mention different styles), it would not surprise me if individual understandings varied. While the understanding by others differ somewhat from mine, I cannot really disagree with them; I find them to be reasonable interpretations. The following quote from Zhang Yun also differs somewhat from my own understanding, but I also find it to be interesting and reasonable (and not something that I would disagree with, even though it is not quite the way that I currently understand the terms):
The meanings of Zhan, Nian, Lian, and Sui are similar at some points, or we can say there are some parts mixing or overlap. So that for understanding them clearly, we need pay more attention to what is difference between them. The main difference between Zhan and Nian is that with Zhan you should make your opponent to stick to you by himself, but with Nian you should stick to your opponent, let him feel bad but can never leave you off. The main difference between Nian and Lian is that with Nian you should always follow your opponent and at the same time make trouble for him, but with Lian you just follow your opponent and do not try to unsettle him. The main difference between Lian and Sui is that with Lian your following like to chasing your opponent, you should always keep touch him and never let he go away. With Sui your following like go away (does not mean losing contact) from your opponent, although he can always touch you, he can never really get you. The main difference between Sui and Zhan is that with Sui you should follow your opponent's active action, but with Zhan you should follow your opponent's reaction which is his passive action.
I would welcome more discussion of these terms.