Based on the description given
here, it sounds like after a few seconds of opportunistic stabbing it turns into a contest of strength, with each shield bearing soldier being assisted by at least one back ranked soldier and opposed by the soldiers on the other side of the shield wall.
If your PC is the one holding the shield, maybe focus on fatigue loss from the extended push/resist push actions. I think there's a side column somewhere called "lending a hand" that talks about multiple characters doing combined feats of strength.