A RT-10 Solid Fuel Booster
Explosive decoupling is a type of decoupling without an decouplers. It is using a thruster set above another to overheat it and cause it to explode, decoupling the segments below it.
It is particularly useful in the start of the career mode in version 0.22 when there is no decoupler. Usually a RT-10 Solid Fuel Booster is used, as it is available from the beginning.