Blocking a finished block to enlarge it or shrink it
 

I understand that one can TEMPORARILY change the size of a block/unit by blocking it.

What happens when the item is washed and the fibers revert to their "normal" length?

Think/Remember - the old denim jeans that we had to wear for a day or two before they were more comfortable and stretched out. Every time they were washed and dried, they reverted back to their "normal" size.

Especially if the unit is NOT quilted to a stable batting - - - - which some say will stabilize a block.

I often have the same thought. I'm guessing that the quilting will hold it in place, sort of like when folks don't prewash...

That would be interesting to test, but I have never been able to block a unit correctly. I've only tried blocking a few times and every time it became distorted.

Sharon Schamber has several videos on Youtube about how to do this when you have a "wonky" block. The purpose is to straighten the block so it can be accurately sewn to other blocks in the quilt top, and so the quilt top will lie flat for sandwiching. If you were to wash the block before it was quilted, it would revert to its distorted shape. However, once it is quilted (at least moderate quilting -- not lines 10" apart), the batting is stronger than the fabric and will keep the fabric from going back to its unstraightened condition.