15:18 What is the other diver doing to the demonstrating diving? Is he doing a quick pull on the demonstrating student's lower BC dump? Seems like a possible unexpected event to keep the diver on point to see how he reacts? Then this laughing or chug chug noises bing made?
Yes, you're right. You're also supposed to push rhs sm.b away from you a bit and give some slack on the line before releasing but I forgot that in the video, too
You didn't purge your backup before starting your valve drill. You also didn't purge your backup after turning your left valve back on. Start with a purge and end with a purge...