Users can crop, zoom, or change aspect ratios (like fixing vertical phone videos for a horizontal broadcast) in real-time.
You can record a live segment and simultaneously play it back on another channel—perfect for instant replays or highlight clips during a long stream. YouPlay for Youtube
It can be triggered directly by your video mixer via LAN or IP. Users can crop, zoom, or change aspect ratios
See how YouPlay manages live production and multi-channel playback in these technical walkthroughs: See how YouPlay manages live production and multi-channel
While YouPlay isn't a "YouTube-only" app, it is a staple for professional YouTube creators who use hardware mixers (like the Blackmagic ATEM).
It supports nearly all video formats (MPEG-4, Apple ProRes, DNxHD, etc.) without needing time-consuming transcoding.
Note that there are also unrelated mobile "YouPlay" apps on the Apple App Store designed primarily as ad-blocking browsers for personal YouTube viewing.