In theory, repel is a poor mans Awe that only works once per turn and only against people with shorter weapons that you can also hit. In practice it is bugged and you should probably just go with the weapon that causes more damage instead.
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Thanks, I didn't know about the once per turn part!
I ran some tests, and it does not seem very effective. Even with fear spam to lower enemy morale, I noticed only a small reduction in hits on my pikemen.