put a block of wood on the seam after they ironed it to weight down the seam while the fabric was cooling
This is called a Clapper. It is a must have tool in the designer clothing and tailor shops.
Cupping is when you have a block that looks convex when it should be flat.
When I was in block exchanges, or putting together blocks from other quilters for a raffle or gift quilt this ruler was a must when the blocks were off from the original size.
http://www.nancysnotions.com/product...marker+tool.do