Help please...crocheted slippers

I am making a pair of slippers (my first time at a clothing item) and I
don't understand the Finishing instructions.

FINISHING
Join pieces as follows: Place Top on Sole with wrong sides together.(I
understand this part)
Join yarn at Heel; working through both pieces, slip st in each sc of Top
easing in Sole fullness; join to first st; finish off.


As I understand this, I work up through the bottom (Sole) to the top (Top)
and slip stitch in the top. What I don't understand is the "easing in the
Sole fullness".


Very thankful,
Christy