The topic of striking a shield with your hand to maneuver it came up in the FS&D forum. One effective way for a hand-matched single sword fighter to manipulate his opponent's shield is simply to punch it in whatever direction is inconvenient for his opponent.
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Todo raised the excellent point that our rules explicitly allow kicking shields, but make no specific mention of punching. My thought would be that if you're allowed to rocket your combat-booted foot at my shield, then my padded hand hitting yours is perfectly fine. On the other hand, maybe not? What's the general consensus on this?