Simply put, sub-ohm tanks are any atomisers that utilise coils with a resistance of below 1-ohm. They are designed to produce intense flavour along with big clouds. If your coil resistance is low enough and you have a powerful box mod, sub-ohm tanks can allow you to vape above 100 Watts. Vaping on the higher wattages allows you to blow bigger clouds and a lot of vape tricksters will utilise sub-ohm tanks to practice and show off their tricks.
Sub-ohm tanks provide a direct to lung hit and they are like smoking a shisha pipe in terms of their draw. This is because of their larger airflows and lower resistance. If you want a more cigarette style draw, mouth to lung tanks will be better for you because you vape them the same way you smoke a cigarette. They tend to have a tighter airflow and higher coil resistance. You can also vape medium-high nicotine strengths in them.
High VG E-Liquids (At least 60% VG & 40% PG) should be used in sub-ohm tanks and you should never take your nicotine levels above 6mg. This applies to both the nic salts and freebase nicotine. Using nicotine levels above 6mg can cause you to cough after every vape, experience harsh throat hits and cause headaches. As a result, sub-ohm tanks are not suitable for new vapers who are coming off a large consumption of cigarettes per day, those who vape high nicotine strengths and those who want an intense nicotine hit. Once vapers can bring their nicotine strength down to 6MG or become less nicotine dependent, they can make the switch to sub-ohm vaping.
If you are new to vaping and you are not a heavy smoker (less than 5 a day), you can go straight to sub-ohm vaping if the bigger clouds and better flavour is more appealing to you.
You must never vape 50/50 e-liquids through a sub-ohm tank. Their higher PG content makes them thinner in nature and they are likely to cause harsh throat hits and your sub-ohm tank is likely to start leaking, spitting back and burn out.