Noob here: so how is this suppose to... feel?

[Pugsy]

Flex is just a little feature to help the breathing feel more "natural" when inhaling and exhaling.
There's a little drop during the exhale phase when Flex is used but if a person is already using 4 cm (either as a ramp starting point or the minimum pressure) then there's no where to drop that 4 cm to for Flex to work its magic.

With Respironics machines they have always needed at least 6 cm pressure for Flex to have somewhere to drop. Hence the fact it won't engage until the pressure is at least 6 cm.
You don't have to use Flex if you use the 6.5 cm minimum pressure...it's a comfort feature that is optional. I only mentioned it because it can make a person a little more comfortable with the inhale/exhale against/with the pressure thing.
At some point later you can always try with Flex off if you wish. I was just thinking that it might make the adjustment a bit easier and less stressful for you.