rajul -

The 'shucks' are the part of the foam block you don't use.

Say you have a rectangular block of foam from which a wing panel is to be cut. Call it roughly 4" thick by about a foot wide by four feet long. The hot wire cuts the airfoil shape, top and bottom. Now you have three pieces:
The wing core itself, and the top 1/3 and the bottom 1/3.

By laying the core in part of the foam block from which it was cut as you work on it, it will support it and prevent you from sheeting in a warp. The foam 'cradle' also helps in keeping it from getting it dinged up on the workbench. Ascii art:

___________________________
| _____________top 1/3 |
|----- (_____ core ____| ------------|
|_____________bottom 1/3____ |