T6 Briarless Mechanic

[Merylx]

I've just returned to the game after three months, and thought I'd give new coils a go. Some of my old friends said they could get me into a static with their alts, and so I happily agreed. Last night we were working on T6 for about an hour, and it seemed like a fun fight. I enjoyed healing it - however, and this is what I need advice on: our static does the MM Briarless method, wherein if you're marked, you run to the main tank and then run through Raffy just as he turns around. This may seem quite laughable, but I'm having problems with this very mechanic. What I'm trying to ask is: what is the exact and literal moment I'm supposed to run through the boss? Whenever I see my yellow sunshine marker, I run to the main tank, and then somewhat 'guess' when he's about to turn around and thus run through. Is it at the exact moment the yellow sunshine marker disappears that I'm supposed to run to the other side? Any advice would be pretty damn good