I'm also curious to hear the answer to this question. Last time I applied, it was only with an idea that had not yet been implemented. Now, I am waiting to see if my current idea has traction before applying to YC. I think they see traction as a very important factor in accepting you into YC. I might be wrong though.

I have no experience with HN personally but I remember pg saying they tried to ignore early signs of traction, as there's no correlation between how far a company is when it's accepted, and how well it ends up doing.

I can't find the comment now, but here's at least half of it: https://news.ycombinator.com/item?id=6897904