The mega landing allows for swingthroughs into btwist and aerial axis tricks. It is not uncommon for btwists from a mega position to be labeled missleg btwists, rather than swingthroughs, but there is a growing understanding that the leg still travels relatively straight through without interruption, rather than stopping to rebound as it would in a missleg transition.
The main key to this transition, as usual, but particularly important for mega, is commitment. Neither the typical landing, nor the swing itself is as natural as some other swingthroughs, and as such can be slightly less forgiving when inefficient. When performed properly, swinging from Inside-Mega positions, such as from a Mega-Aerial is actually quite efficient, but in order to maintain momentum and generate power in this transition, the upper body usually needs to continue past the base leg relatively uninterrupted. One way to accomplish this is to spot forward, which will help to drive the shoulders through the transition, helping to create greater flip for atwists, and greater flatspin for btwists. Simultaneously, the swinging leg is ejected backward, creating plenty of power and lift for most applications. One common practice in this transition is to essentially squeeze the legs, almost resisting the swing itself, and creating a tension, that when released, catapults the following trick into the air.
Swinging from a Frontside-Mega utilizes a traditional frontswing motion, and will most typically be reserved for Webster type tricks.
In the clip above, notice the swingthroughs from both the mega-aerial at the start, and the mega-raiz. The swinging leg is pushed back and up as the torso dives through the transition. Also notice that the mega landings carry quite a lot of flip; the added flip helps maintain momentum, but also gets the shoulders above or ahead of the base leg. Landing short in megas is a recipe for knee problems, so please be careful when trying this.