It occurs to me that this might be a little creepy to some people, so please stop here if you might be easily creeped out.
I can't believe I'm sharing this!
Here's what my daughter does, and what I so clearly remember doing to my mom and grandma:
She pushes the vein across my hand, where it will stay if I keep my hand still.
And then she moves my fingers, which makes my vein slide back across my hand to where it started.