Mouth-to-lung devices are those that allow for a two-stage inhale. Similar to smoking a cigarette, the vapour is pulled into the mouth, and then inhaled. Those transitioning from cigarettes often find a familiar sense of satisfaction with this method of inhale, and so they are most often recommended to beginners.
On the other hand, direct-to-lung devices provide vapour that is inhaled to the lungs all at once. These devices often produce more vapour than mouth-to-lung devices, and are often more attractive to hookah users, those who are looking for a stronger hit with less nicotine, or more advanced vapers.
Choosing The Right Device For You
Style of device is very dependent on your personal needs and preferences. If you are transitioning from cigarettes with no previous experience in vaping or prefer less vapour, it is usually recommended that you start with a mouth-to-lung device (e.g., pen style, low wattage device).
If you are a shisha smoker, have had previous experiencing in vaping, or prefer larger amounts of vapour, a direct-to-lung device may be more optimal for you (e.g., higher wattage devices with sub-ohm [less than 1 ohm] coils).
Take a look at the video below for more information on mouth-to-lung and direct lung devices: