Blood, Unholy Focuses dark power into a strike that deals (225% of weapon damage) Physical damage and heals you for 20% of all damage taken in the last 5 sec, minimum 10% of maximum health.
Frost Focuses dark power into a strike with both weapons, that deals a total of (475% of weapon damage) Physical damage and heals you for 20% of all damage taken in the last 5 sec, minimum 10% of maximum health.
Death Strike is a level 55 death knight ability. It is a deadly attack that deals weapon damage and heals the death knight for at least 10% of maximum health.
Other specs use Death Strike to heal when necessary and not as part of their usual damage rotations (e.g. if the healer is out of mana and cannot keep you up). Its main purpose is to heal, not to deal damage.
While tanking, if it is possible to stand idle for a few seconds without losing aggro, it may be best to wait until you take damage before using Death Strike; this will make the heal worthwhile, and spare your healer(s) some time and mana.
Patches and hotfixes
Patch 7.2.5 (2017-06-13): Death Strike damage increased by 10%.
Hotfix (2017-04-26):Fixed an issue where Frenzied Regeneration and Death Strike could heal for more than their intended amounts in PvP combat.
Hotfix (2016-09-23):Death Strike (Blood) now factors in 50% of recent damage taken from players when determining it's healing amount (previously, it ignored damage taken from players).
Hotfix (2016-07-22):Death Strike is now calculated at 45 Runic Power for purposes of abilities that base their functionality off of Runic Power spent.
Hotfix (2015-03-18):[Requires a realm restart] Reverted hotfix from March 11.
Hotfix (2015-03-11):Fixed a situation where Blood Tap (Talent) and Death Strike could interact incorrectly and allow Death Runes to become permanent.
Patch 6.0.2 (2014-10-14): Now causes healing that scales in effectiveness with attack power, instead of based on damage taken in the last 5 seconds. This healing is affected by [Resolve].
Hotfix (2013-10-23):Reduced Death Strike's healing by 50% for Blood Death Knights in Arenas.
Patch 5.1.0 (2012-11-27): Damage received from players (or player-controlled sources) no longer counts towards increasing the healing done by Death Strike.
Patch 5.0.4 (2012-08-28): Weapon damage increased from 150% to 235%. Can no longer be parried.
Patch 4.3.0 (2011-11-29): Now heals the death knight whether or not the attack misses, or is dodged/parried. As a result of this change, Death Strike no longer refunds its rune cost if it fails to hit the target, as the death knight will still receive the healing effect.
Patch 4.1.0 (2011-04-26): Death Strike self-healing no longer generates threat.
Hotfix (2010-11-24):Death Strike now properly heals for 20% of the damage taken within the last 5 seconds.
Patch 4.0.3a (2010-11-23): Now heals for 25% of damage taken in the last 5 seconds, down from 30%, and will heal for a minimum of 7% of maximum health, down from 10%.
Hotfix (2009-04-21):Death Strike will now heal the Death Knight for 5% of his or her maximum health per disease on the target, up to a maximum of 15%.
Hotfix (2009-04-17):The healing a Death Knight receives from damage done by Death Strike has been reduced.
Healing changed to 5% of maximum health + 5% per disease, up to a maximum of 15% with 2 or more diseases.
Patch 3.1.0 (2009-04-14): Increased damage to 75% of weapon damage (up from 60%), reduced healing by 20% to 80% of damage dealt + 40% per disease (down from 100% of damage dealt + 50% per disease).
[Death Striker].
[Mastery: Blood Shield] bonus.
Patch 7.2.5 (2017-06-13): Death Strike damage increased by 10%. 
[Resolve]. 
