What does it mean when down syndrome child slaps head?

I have had a 4 year old with down syndrome for a while now. She will often slap her head repeatedly when she gets angry and starts throwing these horrible tantrums. I usually spank her to get her to calm down, but she keeps doing this! What does it mean?
Answers

Tri-Harder

Not that it's real (c'mon, "I have had a 4 year old with down syndrome for a while now..."? Really?) but hitting teaches hitting. Basic common sense.