Question

A wooden clapper is used to

A. beat the fabric fibers before pressing.
B. force in the steam after pressing.
C. fasten two layers of fabric together.
D. hold an ironing board steady.

Answer 1

Wooden clappers in sowing are used to force in the steam after pressing and to absorb moisture. 

Answer 2

The answer is B. force in the steam after pressing.
 In order to use the wooden clapper, you need to press your seam with a hot iron and produce lots of steam and it will trap the steams in the fabric.
This will make your cloth feels amazingly crisp, flat pressed seams,  unwrinkled and creases.