Damage Reflection

Damage Reflection is a mechanic in which an enemy's damage is returned to them. This can be done by blocking while channeling with Reflection, Amalgam Javlok Magazine Warp with a Sword and Shield, or naturally with Unairu's Void Spines. The angle of blocking by default is 45 degrees, increased to 90 with Reflex Guard. Furthermore certain enemies such as Fusion Moa require the player to either back up slightly or aim slightly upward in order to block their attack up close. Warframe with damage invulnerability can not trigger blocks, as such they may be ill-advised for a Thorns Build.

Synergies
Amalgam Javlok Magazine Warp causes a 6000% Damage Reflection and additionally reduces blocking's channeling cost by 90%, must possess a Shield Item such as Ack & Brunt, Cobra & Crane, Sigma & Octantis, Silva & Aegis

Reflection causes a 96% Damage Reflection

Focus School Unairu "Void Spines" causes a 100% Damage Reflection

Damage Reflection counts as Channeling for the purposes of Dispatch Overdrive

Taunting is great to help Damage Reflection by making enemies target the user, as such Guardian Derision and taunting warframe abilities are recommendable

Shattering Impact procs with Reflected Damage, as such it can also help with heavy armor

Condition Overload increases Damage Reflection by multiples of 60% based on status debuffs on targets.

Due to Thorns build lacking ability to deal with bosses or those who have invulnerability a powerful single target secondary may be advisable or a status secondary for the purpose of Condition Overload

Due to the Nature of a Thorns playstyle, things which benefit from getting hit such as Rage, Hunter's Adreniline, or Arcanes which benefit from parrying or being hit are recommendable

For more control over blocking, one can use Reflex Guard to increase the block radius while Cautious Shot to mitigate Javlok's Alt Fire damage and trigger a more manual block off of it.

Focus School Zenurik's "Inner Might" is useful to further channeling effiency by 60% if using channeling mods, however cost cannot go below 1

As for Warframes, Saryn's Toxic Lash can trigger off Reflected Damage. There are possibly more warframe synergies with Reflected Damage yet to be found.

Needs Confirmation
(Unconfirmed) Reflected Damage counts as Channeling Damage to be modified by all Channeling Mods

{Unconfirmed) Reflected Damage can be healed off with Life Strike/Healing Return

(Unconfirmed) Reflected Damage is affected by Smite Grineer, Smite Corpus, Smite Infested, and Smite Corrupted

(Unconfirmed) Reflected Damage is affected by auras such as Steel Charge.

(Unconfirmed) Reflected Damage is affected by auras which match the weapon type of the enemy

(Unconfirmed) Reflected Damage is affected by combo counter

(Unconfirmed) Reflection and Amalgam Javlok Magazine Warp stacks