lashes work like so:
Example 25% burning lash:
Your physical damage roll pre DR * 0.25 - enemy's burn DR * 0.25 = burn damage.
Imagine an enemy with 5 crush DR and 4 burn DR and you rolled 12 crush damage per DR. Your overall damage would be:
(12 crush - 5 DR) physical damage + (12 * 0.25 burn - 4 * 0.25 DR) lash damage --> 7 crush + 3 - 1 burn = 9 damage (7 crush, 2 burn).
DR bypass like Vulnerable Attack does not work with lashes, only the initial direct damage (in this case crush). This makes fast weapons with low damage (and blunderbusses) worse for lashes and favors hard hitting weapons when you meet foes with decent DR.
Lashes also don't use MIN damage like the initial crush damage would do. If crush DR is higher than crush damage you will still deal a bit on MIN damage. If burn DR * 0.25 is equal or higher than your lash then you do zwero damage.
Multiple lashes on a weapon (from whatever source) do not get added like so: 25% + 10% = 30% lash. Nope, they will get rolled seperately and both have to overcome DR*0.25. That makes lashes that are lower than 25% very weak because they often get nullifies by enemies' DR. Especially with fast, low damage weapons like flails.
In the case of Starcaller (same example):
(12 crush - 5 DR) physical damage + (12 * 0.25 burn - 4 * 0.25 DR) first lash + (12 * 0.1 burn - 4 * 0.25 DR) second lash --> 7 crush + 3-1 burn + 1.2-1 burn = 9 damage (7 crush, 2 burn, 0.2 gets rounded down to 0).
Note that with Scion of Flame you could raise all your burning lashes by *1.20. So in this example with Starcaller you'd have 25% * 1.2 + 10% * 1.2 = 30% + 12% burning lash.
The only exception where lashes get added is the Wildstrike Belt: it simply adds 10% to your Wildstrike lash. Let's say you have 30% shocking lash from Wildstrike Shock and 15% from Greater Wildstrike Shock. The Wildstrike Belt will NOT add a seperate 10% lash but instead give both lashes +10% so they end up like: 40% Wildstrike Shock + 25% Wildstrike Shock. Because of this, the Wildstrike Belt is a MUST for every melee oriented, shifting druid. If you's add Heart of the Storm you'd have 48% + 30% shocking lashes.
All other lashes get rolled seperately against DR. A weapon with burning lash + Flames of Devotion + Intense Flames is NOT one single 100% burning lash, but instead you will have 25% burn + 50% burn + 25% burn. With Scion of Flame: 30% + 60% + 30%. One reason why a paladin should take Scion of Flame...
In case of a Bleak Walker you could have 25% burn + 50% burn + 25% burn + 25% corrode and so on.
Lashes like it a lot if you can lower enemies' DR via spells and abilites like Expose Vulnerabilites, Hel Hyraf, Sundering Blow, Body Attunement and so on.
Raw lashes (wounding for example or the damage from the Blood Testament Gloves) don't care about DR, so they are great against foes with high DR. Sometimes the raw damage (that is based on damage roll PRE DR) is even higher than the actual physical damage. Example: Drawn in Spring vs. a dragon.
Hope that makes it clear.
Edited by Boeroer, 06 February 2018 - 01:06 PM.