@Kitteh_On_A_Cloud Let me give you a Magic in the Realm 101 so that I can help you understand
Magic was once much more powerful and flexible in the Realms until a Netherese Mage developed a powerful spell to steal Mystra's (the goddess of magic) power.
The spell backfired killing Mystra, destroying the Weave, the magical energy network that exists around the world, almost killing the entire Netherese civilization.
The new Mystra limited the way mortals could access magic, making it a very formalized proccess... If you want to use a fireball you have to move like this, speak that and use bat guano as focus material.
That was true until the Time of Troubles. The Wild Magic zones intrigued some mages as efects could happen inside one, without meeting the formal pre-requisites of casting a spell.
Some extremely intelligent mages, studying this effects discovered a way of accessing raw magcal energies, bypassing the formalism of the Weave.
It was hard and took years of study.
What Nahal's Reckless Dweomer does is that, it bypasses the formal requirements of the Weave to draw raw magical energy to the caster that uses his knowledge to shape it dynamically into the effect he desires. But keep in mind, it's just a level 1 spell. there were many better spells available to a wild mage that never were implemented in BG2.
What we see of Wild Mages in BG2 is a subset of what they could do. A Wild Mage powerful enough could cast spells normally in a Wild Magic Zone without being affected by it, could force all casters around him to misfire their spells, eventually had enough control over more refined Dweomers to call forth most spell effects they desired.
And finally to answer your question, a wild mage had to know and dominate everything a normal mage knew plus the magical equivalent of advanced quantum theory. Just in his case the theory could kill him if he misunderstood or lost his concentration.
Let me give you some nice examples of spells that were never implemented:
Wildstrike - the target must save vs spell or every spell or spell-like ability automatically becomes a surge. The Wild Mage's level is subtracted from the surge roll.
Wildshield - the caster is surrounded by a color shield that automatically absorbs 2d6 spell levels cast against him. Wild surges are absorbed as well and count as 1d6 levels of spells.
Hornung's Surge Selector - If a Wild Surge happens, the wild mage rolls twice in the wild surge table and chooses which effect will actually happen. Lasts for 1 surge/ 5 levels or 12 hours.
Spellshape - If a spell is targeted at the wild mage, he gets a save, if successful, he disassembles the magic energy of the spell and uses it to empower one of his own, being able to immediately cast one of his own spells without forgetting it.