Ok I've only tested this on one server, and it was an Antrix server.
Eat a few until you get the "rapid cast" buff. Now anything that has a cast bar is an instant cast.Mount, trade skills, hearth... you get the idea.
Really helpful lvling trade skills. Not so great in pvp due to the fact you have to eat a few to get the buff, and its only a 2 min buff.
Let me know if this works on other private servers.