Multi-core function

So to use this all I have to do is switch on DX12 or do I need to enable something?

Just dx 12 and a GPU that is fully dx 12 compliant for NVidia that would be a Maxwell GPU , Pascal GPU or Turing GPU

I got a Geforce 1070. That good?

Yes a GTX 1070 has a pascal GPU

Just for clarity
750TI and the NVidia 900 series are Maxwell
NVidia 1000 series are Pascal
NVidia 2000 series are Turing