The clenched signifies strength and determination.  Over the millennia, wherever they have been, the Jewish people have been persecuted.  The Roman's exiled us from the Land of Israel, the Spanish Inquisition drove the Jews from Spain and Portugal, across Europe Jews were persecuted because of their faith, the Nazi's brutally murdered more than 6,000,000 Jews during the Holocaust and, since the establishment of the State of Israel in 1948, our enemies have never stopped in the determination and efforts to destroy.  For Jews, the clenched fist against the Star of David send a message to those who sought to destroy us in the past and those who seek to destroy us today that we shall not give in, that we are strong, will remain strong and will not allow ourselves to be exiled or slaughtered as in the past.