I thought the MCAS would typically be activated while the pilot would already be pulling pack on the yoke. So I'm a little confused under what circumstances MCAS would ever activate. Does it activate while yoke is pulled back but the pilot can additionally yank harder to deactivate?

Yes, it typically would. There's something called an aft column cutout switch which deactivates nose down trim if you're pulling back on the yoke with a certain amount of force. They're proposing to use this switch to deactivate MCAS. I'm unsure how much force it takes to activate this switch, but it seems to me that this makes MCAS less useful for its intended purpose, and that's why I said this decision was questionable.