Basic combat objectives

 The abstract nature of classic D&D combat doesn't lend itself well to calling discrete moves and maneuvers. A lot can happen in a 10-second combat round. Combatants trade feints and parries, with weapons, shields, and body parts. They grab and push and trip. They try to pull dirty tricks like dust in the eyes or stomping on toes. They stumble, maybe fall down, maybe get back up. They may get dazed or stunned momentarily, but shake it off in the same round. All of this is distilled down to an attack roll and possibly a damage roll, because it's assumed the ultimate objective is to hurt the opponent. The specifics of how you get to that point are descriptive only, and don't need to be mechanically codified.

I'm not saying you absolutely can't or shouldn't add mechanics for called shots, parries, trips, throws, dirty tricks, and whatnot, but it all runs counter to the idea of abstraction, that the actions of a round ultimately boil down to how much damage is done.

There are, however, times when doing bodily harm to your opponent is not necessarily your primary objective in a given round of combat. You may aim for an entirely different result, such as restraining or disarming. In some of those cases, we absolutely can find room within the framework of abstraction without breaking it or even bending it unduly.

What sorts of objectives might a player want to try that would be permissible without breaking abstraction? I've mentioned a few, and players may think of more. If it's simply a maneuver (e.g. "I try to trip the orc,") it doesn't qualify. If it's an alternative outcome to a round of combat, and it's something you as DM think is reasonable, it probably fits. "Disarm," for instance, isn't really a specific maneuver; it's an outcome a character hopes to achieve during 10 seconds of melee. 

There's no reason we can't use the good old attack roll for these alternative objectives too. Sometimes a standard roll vs. the opponent's Armor Class may be appropriate, but often it's not. Whether the opponent is wearing leather or plate shouldn't have much effect on whether you can successfully disarm him or push him over a cliff or restrain him so he can't run away. However, how good a fighter he is should absolutely affect the odds of success of those things. Comparing the attack rolls of two opponents models that fairly well.

If you're using an "attack bonus" type scheme rather than THAC0 or attack tables, the comparison is simple: whoever rolls highest, after all adjustments are added, is the winner. If you are using THAC0 or something similar, you'll have to resort to "lowest roll that still hits at least the AC of an unarmored person wins." Or something like that.

So, here's the basic procedure: 

You declare a result you'd like to achieve with a round's worth of effort, if you can, and all the scrapping, grabbing, pushing, and scuffling is done with an eye to attaining that goal rather than the default assumption of dealing points of damage. 

If the DM approves, you roll your d20 as usual and add your bonuses and penalties. The DM does the same for the opponent. (Let's assume the "attack bonus" method rather than THAC0 for this.) If your roll is higher than the opponent's roll, then whatever you were trying to accomplish succeeds. 

Naturally, during all this scuffling over 10 seconds of time, there may be opportunities to do standard damage, as well. Since damage isn't your primary objective though, you miss or forego some of those opportunities and only do half damage if your attack roll is good enough to beat the opponent's normal AC. Note that you can succeed at your declared objective but do no damage, or do damage but fail at your declared objective, or succeed or fail at both, depending on how the dice roll. You can also declare, at the beginning of the round, that you specifically want to avoid inflicting damage at all while attempting your declared objective.

A few examples, to show how it works:

Example 1: A 3rd level fighter and a 4th level fighter, both in leather armor (AC 7 [12 ascending]) are engaged in melee near a pit, and the 3rd level fighter wants to push his opponent over the edge. His attack bonus is +1, while his opponent's is +3. Both roll their attacks, with the 3rd level fighter rolling 11 +2 for a total of 13, and the 4th level fighter rolling 9 +3 for a total of 12. The 3rd level fighter succeeds at his attempt, and shoves his opponent into the pit. His attack roll was also good enough to beat the other's AC, so he rolls damage and halves the result. Meanwhile, the 4th level fighter, though he ends up in the pit, also hits his opponent's AC, and does full damage before he topples over the edge.

Example 2: A 5th level thief tries to climb onto the back of a raging green dragon. The thief's attack bonus is +3, while the dragon has +8. They roll; the thief rolls a 14, +3 for 17, against the dragon's 7, +8 for 15. The thief's roll isn't good enough to beat the dragon's AC 1[19], so she scores no damage, but succeeds in scrambling aboard and hanging on -- for now. The dragon, meanwhile, loses the battle to keep the thief off its back, but does hit her AC of 5[15], and does normal damage. 

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You might wish to apply bonuses or penalties for outcomes that are especially easy or difficult to achieve. You can also require particularly difficult or impactful objectives to be attained in stages; for instance, an opponent must be successfully restrained so it can't break away and run in the first round, then restrained so it can't attack for damage (but can still make attack rolls to resist the opponent) in the second round, and finally pinned so it cannot resist further in the third round. There are many possibilities, for which case-by-case rulings are probably more appropriate than hard-and-fast written rules.