As each plague is read a drop of wine is taken from the cup using the small finger.  Each drop is then deposited onto the plate.  This is explained as an expression of sorrow for the  pain suffered by the Egyptians from each  plague.

The use of the finger is a reminder of the verse in  Exodus 8:15, where Pharaoh's magicians were unable  to duplicate the miracles performed by Moses.  They had to admit that it was the FINGER OF GOD that  executed these miracles and ultimately made the exodus possible.

These are the ten plagues which our Most High God brought on the Egyptians:  BLOOD, FROGS, VERMIN, FLIES, PESTILENCE, BOILS, HAIL, LOCUST, DARKNESS, SLAYING OF THE FIRSTBORN.