Hey guys, noob here. My question is regarding ECS Off Takeoffs. I understand that if the APU is left on for takeoff with the ECS Off, the APU will feed the packs and therefore be a normal takeoff as far as pressurization goes. If you have the ECS Off and the APU off, during takeoff the will turn off and you'll get a cabin bump. What exactly causes the cabin bump? Obviously the packs aren't regulating the pressurization in the initial climb, so is the bump caused by an increase in cabin pressure from air entering the ram air vents?

The outflow valve regulates cabin pressure.
Regardless, this “bump” you refer too is the emergency ram air valve opening due to the packs shutting off (b/c of no bleed source), and the airplane being below 25000ft.

Thank you!