Exposing brain material supposedly gets them to eat the rat or mouse. I don't think it matters if it's fresh kill or f/t.

I've never personally had any issues with switching back to rats from gerbils but I could easily see that happening. I've never actually had any luck with f/t and balls so I've always had to do live or fresh kill. I gave up even trying f/t when I only had balls because it was a waste of time and money.