I like that exercise! thanks for posting it here! definitely can feel my serratus activating, especially on way up, on way down not so much.
I have just finished reading Evan Osar’s book (flats you linked one of his videos) it’s called Corrective Exercise Solutions to common hip & shoulder dysfunction.
found it really interesting, a lot of physiology that i was not familiar with & techniques that i can already see some progress with.