Pneumatic hammers as a sledgehammer enhancement. Costs x energy per swing, reduces AP cost of hammer attack by 4. The logic here is that the hammer is activated when you hit your target, generating force in the opposite direction to bounce your sledge back into swing position.
A 20 AP tungsten sledge (belt + non-tabi boots) would drop to 16 AP, for 3 attacks per turn
A 16 AP tichrome sledge (belt + tabis) would drop to 12 AP, for 4 attacks per turn
You aren't getting the extra electricity damage + AoE, but you get more swings per turn, possibly doing more damage with crits or damage-stacking (bone breaker, tfb, toxic gas, etc)