Because of the way that you’ve laid out the bills in the beginning, the bottom bill will roll-up earlier than the top bill. When the bottom bill is completely rolled up, you’ll notice that its exposed corner will “roll over.” This is the moment when the bill on the bottom becomes the bill on top.
When the bottom bill “rolls over,” it helps if your right hand covers the action. Just roll the bills with your left and right hands and use the fingers of your right to rest on the table and cover the corner of the bottom bill. (For illustration purposes, the right hand is not shown in the pictures.)
After the bottom bill “rolls over,” you can ask the spectator to place their fingers on each exposed corner of the bill to make sure that nothing happens to them. Unknown to the spectator, the bills have already traded places.
