Improved Rend increases the bleed damage your Rend ability by up to 20%. It is a passive Arms talent, and automatically affects the Rend ability when it is used. It has no prerequisites and is thereby a cheap talent in itself.
Rend is rage efficient DPS boost as it is, and this talent improves, indeed, on that. It allows Rend to be efficient even in battles that doesn't last the duration of the Rend. Because both the base damage and the additional damage is multiplied, the advantage of slow, hard-hitting weapons are even greater.
However, it still isn't usable in Berserker Stance, and it is still only possible for a warrior to have one Rend on a target active. Because of this a warrior may only have the opportunity to deliver one or two rends on a target before the battle is over, and this means an investment in rend needs to take into account how much it is used compared to, for instance, [Heroic Strike]. While [Heroic Strike] is worse than Rend from a purely rage-to-damage viewpoint, it is among the only burst damage and high threat attacks available at low levels. A low level warrior going the way of the Arms may choose to invest in improved Heroic Strike early and re-invest in Rend later when Heroic Strike is used less often.
[Rend]
[Heroic Strike]. While 
Patch 4.0.1 (2010-10-12): Removed. 
