T4 powder special ability

Does the special powder ability thing automatically activate or do i have to press a button

 

You have to wait until the indicator above your hp and mana bar says 100 percent, then use your main attack (right click on archer, left click on every other class) while pressing shift.

 

also make sure there are two t4 powders of the same type on the item because if you only have one it won't work