I created a Hybrid tutorial discussing Contribution Technique in the context of Problem E. Iris and the Tree from last Codeforces Round. The video just goes over key insights, if you want to dive deep, you should read the blog after watching the video.

Video Link : https://youtu.be/xWGFdf2pvh4?si=dm6GZ0BWPsFP0IU1

Blog Link : https://cfstep.com/training/tutorials/general-techniques/contribution-technique-on-trees/