I think that'd be the focussed witchfire rules.
If a power is focussed witchfire and you roll under 5 on your psychic test, you can choose the model you hit. If you roll above, it's randomly determined.
However, if you use puppet master against a single model unit (like a tank) it can only be allocated to one model, so it hits that model.
So puppetmastering a tank is fine.