Drona devised the Chakravyuha to capture Yudhishtira, as a prisoner of war. Such deadly was the trap of Chakravyuh that only talented warriors like Krishna, Arjuna, Drona, Bhisma and Pradyumna knew the way to come out of it.

How a Chakravyuh works:

~there are 7 layers (SS1)

~Soldiers of the inner level are stronger than the soldiers of the immediate outer level (so 7th layer has the strongest warriors)(SS2)

~infantry made outer layer & archers on chariots, cavalry & elephants made the inner layer

~layers should not break

~to break the formation, one needs to divert attention of archers in inner layers
~The whole formation is not a defensive strategy but an offensive one. this means that the Chakravyuh
~movement of soldiers in one ring is in opposite direction to that of another

~the arrangement/direction is changed by commanders using conch/drummers
~killing unarmed drummers is unethical
~The Vyuh is recognisable only from above the ground & not from the ground.
~enemy soldier is to enter from the mouth after which mouth closes to prevent backup

~due to circular motion he gets trapped
~ideally he should not survive until he reaches centre
~those who attack from outer layers will eventually enter the mouth and get similarly trapped in a maze
~each layer drains the energy of an enemy soldier

how to break the vyuha?
for finding shortest possible route to centre one has to look for weak points and break the layers.

Abhimanyu broke it but couldn't exit

So it acts like a filter. The main warriors don't lose their energy fighting the less mighty ones

this is brief explanation. more deep info here. (pics are from internet)
http://mahabharata-research.com/military%20academy/the%20mysterious%20chakravyuha.html