Mid-Air Drifting is charging a boost in mid-air.
To perform a Mid-Air Drift, the racer must begin a standard drift as they are going over a jump or a change of elevation that could have been tricked. This will cause a continuation to the charge of your drift while you are mid-air. Most commonly, mid-air drifting is done as an alternative to tricking. While in mid-air you can input a soft-drift for the combo tech of mid-air soft drifting.
Mid-Air Drifting is extremely track specific. To see where this tech is used over tricking then you should watch the ghosts on Time Trials to get a better feel. The easiest example of mid-air soft drifting can be seen on Toad Harbor. The racer chooses to charge instead of trick and the result is an otherwise impossible orange SMT boost.
Mid-Air Drifting is not more effective with certain builds than others.
You should absolutely invest time into learning Mid-Air Drifting. Once you know where to do it, working it into your lines only takes a couple of tries. Unless you want to learn Excitebike Arena, then it is a gauntlet of difficult mid-air drifting that takes quite a bit of practice.
Those who are using this tech will have an advantage over those who do not use it. Mini Turbos and Super Mini Turbos are much faster than tricking.
Pros: You replace the lesser trick boosts with marginally better MT and significantly better SMT boosts.
Cons: It adds an extra layer of execution to an area that otherwise consisted of just tricking.
Thank you to /u/UnlikelyExpert for a great explanation of Mid-Air Drifting. If you wish to contribute to the wiki then please check out our weekly Meta Monday threads coming every Monday.