Cannot be Special Summoned. Requires 3 "Qli" monsters to Normal Summon/Set. If this card is Normal Summoned/Set, it is unaffected by Spell/Trap effects and by activated effects from any monster whose original Level/Rank is lower than this card's
current Level. Once per turn: You can target 1 monster your opponent controls; take control of that monster until the End Phase.d

Yeah so I was just wondering... Considering that last effect, could you use it on the opponents turn and then use the controlled monster's effect? I'm thinking maybe in a Judgment Dragon scenario, ie: take control of it, pay the 1000 LP and then nerf the field, leaving Kernel safe?
Was on Duelling Network, this happened to me, I left slightly confused.
I just got back into Yu-Gi-Oh and just started playing with a Qliphort deck.

I'm a bit late, but here's your answer:
The wording "Once per turn:" means the effect is an Ignition effect. Ignition effects can only be activated during your turn, when the gamestate is open, ONLY as Chain Link 1, in your Main Phase 1 or 2.
You cannot use ignition effects during your opponent's turn.
Here's the Wikia's definition of Ignition effects:

Post-Problem-Solving Card Text, these type of effects will generally be indicated by "Once per turn:", "You can" or "During your Main Phase:", followed by the cost which will end with a semi-colon, and lastly, the actual effect which you activated.

Judgment Dragon's nuking effect is also an Ignition effect, which means it works the same way as Kernel's.
To add more, Ignition effects are Spell Speed 1 effects (which is why they can only be activated manually ONLY as Chain Link 1 and during an open gamestate). Since you just got back to the game, I'll go ahead and provide you with the definition of this as well:

Spell Speed 1 is the slowest of all Spell Speeds. These card effects cannot be Chained to the activation of a card or effect. Typically, these effects cannot be Chain Link 2 or higher, unless multiple Spell Speed 1 effects are activated simultaneously.

Tl;dr: No. You cannot do that.

Solved.