The reinforced line consists of two layers of units staggered to allow the back units to freely reinforce the front units. Reinforced lines tend to used with very balanced lists, but this in not always the case.
Reinforced lines are usually designed with two types of unit that compliment each others weaknesses. In most situations you have a lock and a pop unit. Usually the forward units are the lock units. They tie up any incoming units and the pop units (also known as a counter assault units) come in to destroy the units that are locked up. As good example of this list is a hth heavy Imperial guard list that uses shooty infantry units up front to bait units charge them in hth then charges in with units of rough riders, ogryn, arcoflaggelants etc to clean up the locked up unit.
Similar tactics can be employed with armies like Eldar with guardian squads and howling banshees. This would be an example of a defensive reinforced line which uses shooting to force engagement.
The strength to this formation is that it uses very specialized units to work together in tandem to take down units that they could never handle on there own. In essence, the two units combined are stronger than the two units of their own. It also allows you to set up situations where you know certain unit will excel. This is a key point if you play a specialized list like a Beil-Tann list. Aspect warriors an unstoppable in the situation they were designed for, but tend to flounder and die horrible deaths if caught in a bad situation.
This formations weakness lies in the fact that the two different types of unit have to compliment each other. If your opponent can kill off most of the lock or most of the pop units. The left over units tend to be too specialized to survive on there own.
The previous example was for an army on the defensive with shooty units up front anchored by counter assault units behind. One varitition is what you see with horde armies like Tyrannids. Fast lock units like hormaguants run a head to tie up units, only to be followed by a second wave of pop units like carnifex, hive tyrants and warriors.
Another variation is what you see with balanced space marine armies. The lock units are actually deployed in the rear and consist of units like devastator squads and other shooty units. They lock down opponent in area of terrain by taking away movement lanes allowing mobile pop units like assault marines, dreadnoughts etc to engage units in hth.Comments
The reinforced line is similar in use to the balanced line but tends to be composed of units pushed to either side of the spectrum of shooty to hth. Really the important take away is the concept of deploying troops to set up intentional counter assault. It is an important concept in 40K as it allows you to dictate your opponent's movement.