Y-Combinator's policy requires all notification to be physically delivered - i.e. US Postal Service or UPS or another delivery service is acceptable. Email is not acceptable - and presumably fax is not either, since no number is listed.
This means:
1. It will take several days for most takedown requests to be acted upon, by which time material will probably have rolled off the front page.
2. There is significant documentation when an entity pursues takedown in less than good faith - and if it came via USPS there could be substantial penalties for any fraud.
3. It makes bulk takedowns more difficult because each notice is required to provide adequate documentation.
In other words, Y-Combinator places a meaningful burden on those claiming violation of copyright, and I suspect that this creates enough friction to eliminate many claims - making HN better, and letting YC staff do more important work.