You start out very slowly, sit with your eyes closed for a while, and slowly lift your arms, not so much in front of you, but with your arms slightly open, at an angle. When I discovered this (quite by accident, before Robert wrote about this) I was meditating, when I thought I could see the room in my perpheral vision, as if I had two patches in front of my eyes but still could see through the edges. This is what made me lift my hands to see if I was actually seeing what was there and not some werid brain memory thing. And I was surprised to see I could see the impressions of my hands and arms (and legs if I was in the correct angle). Moving my hands clinched it.