Ask HN: Why is YouTube adding “&pp=sAQA” to video URLs?

YouTube started adding a new parameter "pp=sAQA" to video URLs on most index style pages (e.g. /feed/subscriptions, search results, the /videos page on any channel). The actual video pages (/watch) strip the pp= parameter, and it doesn't appear to be added to the URLs for the "recommended" videos.

Does anybody know what this parameter does and/or why it was added? It's really annoying; using YouTube URLs in the shell now requires quoting due to the "&".