From the recent leaks I was able to find out what Overpower does and its rune effects.
OVERPOWER
Press the advantage of a powerful attack to overwhelm the enemy for X weapon damage to all targets within X yards. Overpower becomes available whenever the Barbarian makes a critical hit against an enemy.
-RUNES-
A Killing Spree: Whenever used, your chance to critical hit is increased by X for X seconds.
B Storm of Steel: Throw up to X axes at nearby enemies which inflict X weapon damage each.
C Revel: Heal X of your maximum Life whenever Overpower is used.
D Momentum: Overpower stays active for a duration of X seconds and generates X Fury for every enemy hit.
E Crushing Advance: Reflect X of incoming melee damage for X seconds after Overpower is activated.
OVERPOWER
Press the advantage of a powerful attack to overwhelm the enemy for X weapon damage to all targets within X yards. Overpower becomes available whenever the Barbarian makes a critical hit against an enemy.
-RUNES-
A Killing Spree: Whenever used, your chance to critical hit is increased by X for X seconds.
B Storm of Steel: Throw up to X axes at nearby enemies which inflict X weapon damage each.
C Revel: Heal X of your maximum Life whenever Overpower is used.
D Momentum: Overpower stays active for a duration of X seconds and generates X Fury for every enemy hit.
E Crushing Advance: Reflect X of incoming melee damage for X seconds after Overpower is activated.