Hey there Eric, I am yet to do Synthetic vs Sympathetic but the essence of it is: is your leadpipe sound Sympathetic i.e. free, loose and in response to the the reflecting pressure wave OR Synthetic i.e. pinched, manipulated, artificial.
I am going to do a video going through the entire “Dispelling Brass Playing Myths” presentation. It is on the vast list! But don’t panic, all of the information is covered here in the course.