Defective Doomsday Device
Source: Profession. EDIT: As of 10.1.5, it appears that the drop rate of the toys has been increased to 10%.
The 
Defective Doomsday Device is a throwable toy obtained through Engineering by using Scour Through Scrap after maxing out both the Inventing and Scrapper specializations in The War Within.
Guide awaiting review
Max Inventing and Scrapper Specializations
Before the Defective Doomsday Device can drop, you must fully max out both the Inventing and Scrapper Engineering specializations. You will not have a chance at the toy without both specializations maxed.
Farm Scour Through Scrap
Use Scour Through Scrap repeatedly on 
Piece of Scrap to generate chances at the toy. As of patch 10.1.5, the drop rate for the toys from this activity was increased to approximately 10%.
Reset Cooldown with Invisibility Tinker
To farm Scour Through Scrap faster, enter the first wing of LFR Eternal Palace, use the invisibility tinker item to pull the boss, then reset him and activate the invisibility tinker again — this continuously resets its cooldown, letting you scrap repeatedly.
Use the Toy Quickly
Once obtained, the Defective Doomsday Device must be thrown at a friendly player while standing close to them. If you wait too long, it explodes in your hand and you receive the buff instead.
Tips
Use this macro to quickly consume Piece of Scrap without opening bags manually:
/run for bg=0,5 do for s=1,C_Container.GetContainerNumSlots(bg) do local lnk=ItemLocation:CreateFromBagAndSlot(bg,s) if lnk:IsValid() and C_Item.GetItemID(lnk) == 198651 then C_TradeSkillUI.CraftSalvage(382374,1,lnk) return end end end
