It is effective to use Rend in a rotation in if your rage generation is very generous. This could include having great combat ratings and powerful autoattacks, or being fed rage through taking damage. Rend is used after the 2nd BT if there is no Bloodsurge available. There is a stance change to Battle Stance, so rage will drop down to maximum of 25 with the Tactical Mastery talents, Rend is used and you will return back to Berserker Stance with that maximum. It is very possible that your autoattacks will land within the 1s you are in Battle Stance, so a stoppage of Heroic Strike is required while in Battle Stance, and a time after the return to Berserker so that you don't accidentally starve yourself of rage to where you can't use BT. Since there are no Heroic Strikes for a time, there is no chance for a new Bloodsurge.
Rend can be anywhere from 1.5-2.0% representation for a gain of about 1% overall damage unglyphed, and 2.1-2.8% representation for a gain of about 1.5% overall damage glyphed. Glyphing Rend would replace Glyph of Execution in single-target only scenarios. It is recommended that only very advanced players utilize this in their rotation due to its small net gain, and very large penalty if mishandled.