The Nipple Sandwich is a technique one that helps make latching-on easier for some mothers.
A mother basically use the C-Hold or U-Hold & gently squeezes her fingers & thumb slightly together (sort of "flattens" the areola area)... mother pushes in toward her chest wall or ribs... then, she pushes in with her thumb more than with her fingers. Mom then encourages baby to open wide & pulls in close...
I'm sure the LC's you're working with can show you some other literature... I'll look to see if I can find some, too.