He explains it earlier in the thread. Its over a couple of posts though so I'll summarize. The point of precasting a shot is to give you more globals during the fight which you dont' get casting a 50 focus Aimed as you'll lose the extra global you got regening with ST. GT on the other hand costs a lot less focus and begins regening focus passively as soon as you cast. As for precasting aimed giving a better chance at a TotH proc that is true, you are 21% more likely to have a TotH proc when precasting (48% vs. 69%) but you'll only have ~16 focus to spend on the proc unlike the GT opener which would leave you with ~54 focus allowing you to dump 2 TotH Aimed Shots before needing to ST. Even if you don't get a TotH with GT, Aimed, Crows you can still cast a 50 focus Aimed afterwards fishing for a proc, only it'll be under the hasted effect of RF and affected by all of your procs.