Ask HN: When does YC contact references during application?

Wondering at what stage YC will contact current employers about a reference check during the application.

1) Is it when they are evaluating the initial application submitted online?

2) Is it after the application is accept and before the interview stage?

3) Does YC respond to requests not to ask for reference checks? Worried about putting my part-time employment at risk during the process.

Thank you,