I am a little bit confused about the combat rule concerning rear attacks from the Player's handbook. If I have enough movement, why wouldn't I go straight to the rear side of an opposing creature to gain that nice +2 bonus to my attack roll (Revised, p. 180; original, p. 90)? Humans, Elves and Half-Elves have plenty of movement, why would they ever stop before they reach the backside? I'm also not entirely sure if you can charge straight to the backside when you're in front of a creature, gaining +4 to the attack roll.
Last one first: you can't “charge” someone's back when they're facing you, because charging has to be a relatively straight, well, charge at the intended target. It's definitionally not charging if you're not running straight at them. A charge can't involve stopping to turn around.
(If you're coming to AD&D 2e from D&D 3.5e, this may seem unintuitive — where does it say that a charge has to be in a straight line? AD&D 2e doesn't work that way: it expects readers to read words to mean what they say. “Charge” isn't redefined by the core game anywhere as something more flexible than its real-world meaning, so it isn't and it has to be an actual, factual charge as we understand it in the real world.)
Flanking and attacks from behind
Movement in combat is limited by two things: your movement speed and engagement with the enemy. As soon as you are within striking range of an enemy you are “in combat”. (Again, this is definitional of the English words that the game expects readers to understand, it's not specially-defined by the game to mean what it already means in English.) The only way to get out of combat is to use one of the retreat movement modes (p. 97 of my classic blue-and-black first printing): fleeing or withdrawing. Both can only be done when you haven't already moved for the turn, and both require moving more-or-less directly away from your opponent.
This has a few interesting, emergent consequences, among which are:
- It is impossible to walk up to someone and walk past them, if they are armed and consider you an enemy.
- Once within melee combat range of an opponent, you still can't get past them no matter how you try to move, since you can only move by going directly away from them.
What this does is make melee opponents “sticky” — once engaged in melee combat with someone, they limit your movement (and you theirs). It makes a front-line of AD&D 2e combatants (durable Warriors on your side, hopefully!) extremely effective at locking down melee enemy movement. This emergent outcome is something that is widely emulated in other games by using more complex rules, but is “free” and relatively simple in AD&D 2e due to withdrawal/fleeing being the only legal movement options once in melee range of an opponent.
Getting around your enemies
So, to get around your enemies, you have to have enough space and movement to move all the way around them without them turning to face you.
That means you either have to
have enough movement and space around the melee to circle wide around all enemies and still reach the desired target's back (unlikely, when the average human/elf/half-elf can only move 60 linear feet in a round and still attack)
have enough space and time to circle wide around them over two or more rounds while they stay facing the way you want them to, either due to your allies keeping them occupied or due to your own stealth preventing them from knowing to turn to stay facing you
In other words, you have to flank them for real, with real tactics and movement. You can't just brush past them, turn, and stab them in the back. In general, attacking someone from behind is either the result of an opportunistic attack on someone who already has their back turned to you for some reason, or something you have worked hard to set up.
The main reason is avoiding attacks of opportunity. You can't pass through an area within reach of your opponent without provoking an attack of opportunity. This means with an opponent with a reach of 5 feet, starting just out of reach you have to travel 16 feet just to go around them. IF you are already in melee you have to leave melee, which again provokes an AoO.
Attacks of opportunity occur when a threatened character or creature ignores the enemy next to it or turns its back on a foe
-AD&D Combat and Tactics
You can't charge through an opponent to get both bonuses because it doesn't make sense. Charge gives you a +2 to attack because of your furious momentum, if you don't use it, charge through and switch direction to stab your opponent in the back it doesn't make much sense to give you that bonus does it. This isn't explicitly stated in the rules but any DM I know would rule 0 it that way. +4 to your attack roll is a lot for no additional consequences beyond that of the charge. At the very least you provoke an AoO by moving through your opponent's occupied space with the full disadvantage that you get from charging.