I was on the Shadow PC for IEEE S&P 2020. Shadow PCs replicate the entire peer review process of a conference in parallel to the actual conference. This includes reading and reviewing papers, discussing them with other members of the shadow PC, and then arriving at a consensus on the papers to be accepted for the conference. The shadow PC then compares its results with the actual PC to see how well it did.

These are my takeaways:

1. Compare and contrast my reviews with those of others over 10-15 papers.
2. See both good and bad examples research papers.
3. Experience the 'backstage' reviewing process.
4. Network with other students and post-docs with shared interests.
5. Network with the host institution.
6. Free travel. This year it was hosted at Google, NYC.