Let me take an example from Hannibal's awesome raiding video.
As you can see
1st and 2nd wall breaker is use to shorten the length of the spikes
3rd and 4th wall breaker is use to "set" the direction of pathing for wall breakers that are subsequently deployed at that position
5th and 6th wall breaker was deployed even before 3rd and 4th blow off in order to have that over-loading effect.
7th and 8th wall breakers however, was deployed too early, as for those parallel spikes, we do not need to use the "over-loading" technic to tunnel through. Hence the target timer of the wall breaker resets, and aim for the closest wall to them instead.
9th and 10th wall breakers was deployed after 5th and 6th blew up, hence they can tunnel through perfectly.
Awesome work hannibal :o
http://www.youtube.com/watch?v=6Dv5zQL0NxQ