Teetime

I am new to the d750. When shooting action video, what is the best way to get focus? I've seen reference to "1:1 focus assist" but haven't been able to find out how to enact it. I am able to zoom in/out using the two buttons, but is there a more direct way to zoom to 1:1 for focus, then release it when in live view video mode? So far I am quite impressed with the usability of video mode compared to my soon to be sold d600.