The "Refused Flank", or oblique order as it's also called, is a useful tactic when facing a numerically superior force. By concentrating the majority of your strength against one flank of the enemy, you can acheive a local superiority in numbers.

You will need to deploy a few units along your line to bait your opponent into deploying across a broad front. You are intentionally weakening a portion of your line to concentrate your troops on one flank. Units deployed in this 'decoy' duty should ideally be able to redeploy fairly quickly. Tanks or other vehicles work best in this role.

At this point, you can either attack or hold your ground depending on whether or not you have the advantage in mobility. If you opt to attack, you hit one enemy flank with concentrated force, while "refusing" your weakened flank. If you hold tight, you will be forcing your opponent to approach piecemeal into the majority of your army rather then facing off with equal forces.

With either approach, aggressive or defensive, the idea is to defeat the enemy army in detail, killing a unit at a time and working your way up his line. Your units operate in close support of one another, while the enemy on the opposite flank is out of range.
 

The Refused Flank - Defensive

The "Hook" looks a lot like a phalanx deployment, at least until you start placing fast units. Deploy your heavies and troops in normal phalanx fashion, opposite the enemy across a fairly broad frontage.

Once you get to the point of deploying your fast units, much of your opponent's army will already be on the table. By looking at his deployment, you can get a pretty good idea of what he's got planned. You then place your fast attack units to one side of your line, in a position to hit one of his flanks.

The basic premise is to "steamroller" with your main line while your "hook" element speeds in and hits his weak flank. The flanking units attempt to roll-up his line and take out his 'CoG', while the rest of your army moves in to mop up.
 

The "Pincer", or double envelopment, is a classic element of military strategy which has been used to great effect throughout history. At first glance it appears very similar to the "Hook" attack, but there are a couple of differences.

The most notable difference between "Pincer" and the "Hook" is that both flanks of your opponent's army are attacked simultaneously instead of just one. Another more subtle difference is that your fixing force isn't used as aggressively in the "Pincer" attack. The center of your line either holds position or gives ground to lure the enemy in.

Hannibal's double envelopement of the Romans at the Battle of Cannae in 216 BC is viewed by military historians as one of the greatest battlefield maneuvers in history. It is cited as the first successful use of the pincer movement to be recorded in detail.
 

The "Bull's Horns" is similar to the pincer. It is an aggressive tactic pioneered by the Zulu of South Africa. The attacking force is divided into four groups - two in the front, one on the left, and one on the right.

The main group would face the enemy, while two "horns" would engage the enemy on either side and force them towards the center. The fourth party (usually the veterans) remained as a reserve.

Translating the "Bull's Horns" to 40k is pretty simple. It uses a deployment very similar to the pincer maneuver. Fast units deploy on either side to advance forward and encircle the enemy. The center of your line, instead of acting as a lure, like in the pincer, is now the main force behind your attack.