Help creating a custom potion effect

[chimera27]

I was wondering how I would go about making a potion effect do various things to the player, like change their speed, change the damage of certain items, ect. The problem is, there is no documentation anywhere in the vanilla source that shows how vanilla potions actually carry out their effects, just things like func_118237_a. I was trying to make a potion that replaced an item in the players inventory with another and take it away when it wears off, and also make some stat changes. I already know how to do the first part, I just don't know how to make the potion do it, or how to make changes to the players stats. Also there are no tutorials I could find explaining this, just ones saying how to make the effect itself. Any help with this would be IMMENSELY appreciated!

[chimera27]

Anyone?