I think I’ve got the masking part down, but how do you turn one clip into a mask for another? I can Luma Key it but that makes everything too bright, resulting in a purplish-pink lightsaber: https://drive.google.com/file/d/1SrQK7JrJcpPe1zPLndvs_bARQsPHrWui/view?usp=sharing
Edit: I found it, “Track Matte Key”