Any shock based attack. Also if you stack to the maximum amount for stack Debuff, you gain full potential of Electrocution. For example , 4 stack Debuff means you get 50%×4= 200% increased damage from electrocution set, along with 50% DMG from stack Debuff. With shock effect Max, that's 5 shock attacks so you'd get 250% DMG from Electrocution and 62.5% DMG from 5 stack Debuff.
Though it is possible to stack hirling stack Debuff to get even more use from Electrocution and maybe sorcery talent works with it but either way, Electrocution is extremely powerful. Although I also think sorcery talent only works on Damage over Time (DoT) instead of Debuff over Time (DoT) but it's extremely good on poison and fire.
It is possible to interrupt the combo so when you attack with shock and then the Debuff gets replaced by fire dot, then the Electrocution Debuff gets lost. The fire attack does get buffed but then you'd have to manually apply shock stack Debuff again.