Essentially, a more powerful Wizard can spend his turn attempting to counter an enemy Wizard's spells. If he is more powerful than the enemy, he counters the spell on a 7-12 on 2d6. If less powerful, he needs to roll 8, 9, 10 or 11. That works out to around 58.3% and 38.9%, respectively. There's no such thing as equally matched Wizards, apparently; in case of a tie, a die roll determines relative strength.
Obviously, the AD&D rules are more granular when it comes to magic and magic-users, so a tad more expansion is called for if deciding to adapt these rules to AD&D use. I've not playtested this, of course, but I might do something along the following lines.
Any magic-user is able to use the magical energy stored in his mind to attempt to counter the effects of another magic-user's spell. A counter-spell has an effective casting time of instantaneous. The countering magic-user informs the Dungeon Master of his intention to attempt to disrupt the casting of another magic-user (magic-users and illusionists can disrupt one anothers' spells in this fashion, but not clerical spells), and chooses a spell from his own list of memorized spells to use, before the nature of the enemy spell is known.
(In this way, the choice of which spell to "burn" becomes a real decision and something of a blind bid; do you waste a fireball spell when the enemy could only be casting magic missile?)
Roll 2d6 and compare the level of the spell burned vs. the level of the spell being countered.
|Level Difference||Countered On Role Of...|
|-7 - -8||3-12|
|-5 - -6||4-12|
|-3 - -4||5-12|
|-1 - -2||6-12|
|1 - 2||8-12|
|3 - 4||9-12|
|5 - 6||10-12|
|7 - 8||11-12|
* Counting cantrips as 0-level spells, it is possible, however unlikely, that a cantrip could be used to counter a wish spell. Yes, I like cantrips. Wanna make something of it? ;-)
The difference here between this and the system found in Chainmail is not only the granularity of the odds, but also using the level of the spell as opposed to the level of the caster. If you really want to get into it, a modifier for level of caster could also be included, but you'd need to jig it so as to bear the minimum caster level for a given spell level in mind. I would probably give an automatic success if one happened to burn the exact same spell being cast, especially since it's a blind choice.
Playtesting might also indicate that my own table above is too generous; a 7th level spell should be able to squash a cantrip with no chance of failure. I could also see spreading out the energy in the case of multiple spell-casters, attempting to counter several lower-level spells with one high-level spell in a single go. But I'll save that for another day.
This is just a first pass, inspired by that great post. I will drop this into my next AD&D campaign and see how it works.