To my understanding its:
MTL = Mouth To Lung - drawing into your mouth then into your lungs, using the suction from your cheeks
DL = Direct Lung - pulling directly into your lung with an inhale that is basically a slow deep breath (or a big deep breath if your vaping on something with massive airflow!)
The AIO (the one i have is an older one, so if you have visible airflow slots on yours, this may not apply) is well suited for a restricted lung hit - i find even the tightest setting a bit too loose for MTL.
The AIO (and other devices that take replaceable heads) are built to have the pressure from you sucking to pull the juice into the wicking material in the coil head. There should be resistance with these setups, other wise expect dry hits.
The AIO (again, the older version) works best with the 0.6ohm heads - it's built for it. Putting a .5ohm head in gives me a weaker and less satisfying vape. It takes more volts/amps (watts) to push a .5 ohm head than a .6.. seems like the difference shouldn't be noticeable, but in my experience with the AIO it is. Also it has a smaller tank thank most other cubis tanks, so the wicks dry out when the tank gets below 1/4 full as the wick holes on the side of the coil head aren't exposed to the juice. not a huge deal with a 4 ml tank, but with the little 2ml AIO tank i prefer to fill it after the 2mls are basically gone.
With the AIO i usually set it on a tighter air flow and direct lung hit, exhaling through my nose and mouth. pulling harder seems to pull plenty of juice to the wicking material and I don't suffer from dry hits. Occasionally I'll MTL a few hits, but usually that drys it out pretty quick so i switch back to a few DL pulls. I usually vape 3mg in my drippers/subohm tanks, but bump it up in the AIO - 6mg. It's noticeably stronger, but i use my AIO as an out and about device instead of the thing being permanently stuck to my lips like at home with the sub ohm tanks/drippers.
If i'm remembering correctly - the vape molecules are too large to be absorbed by the bronchials in your lungs (which is how nic from cigarette smoke is absorbed) - the glands in your nose and mouth are what absorbs the nicotine in vape - it takes a little longer for the nic to kick in, but you aren't getting all the other crap you get with cigarettes. It takes a little time to get yourself off the other crap in cigarettes (MAOIs...) so most recommend a higher nic to get you over the hump of kicking that crap. Figured I'd throw this in as you noticed a huge improvement when you exhale through your nose - if you want a stronger hit - draw it into your mouth, french inhale (slowly push the vape out of your mouth - partially parted lips while you lift your jaw and tongue toward the top of your mouth - while you inhale through your nose) then exhale out of both your nose and mouth at the same time... MTNTL?
This post may seem a little critical of the AIO, I don't mean to bash it at all - I love the little thing, but like everything it has its strong points, weak points. The way I vape works for me and if something else works for you, keep it up! There is no right or wrong way to vape - it's all about what works for you (unless you are being unsafe, that's the wrong way to vape!).
