Each place holder awards the equivalent of 5 elixirs over the cap. Max is 100 over cap (or 20 place holders), if you are currently undercap, these are applied as regular elixirs.
After receiving the place holder please fill out the appropriate form:
-HP Over-Cap Elixir Form: HERE
-Chi Over-Cap Elixir Form: HERE
Keep placeholders in your Gate Master Bank, after few days, placeholders should dissapear, and overcaps will be done.