They have that Q on their FAQ on the GP site.

from there:>>>

This is a small balsa stringer that is simply cosmetic. Rather than having a hard edge where the sheeting ends and the stringers begin, these shapers simply create a smooth transition from the sheeting to the stringer-only look at the back of the fuse. Simply sand the shaper into a soft gradual taper into the stringers.

http://www.greatplanes.com/faq/faq-gpma0250.html