What you need to do is open a TiddlyWiki as a text file and search for this string (regex search)
exports\.[a-z]+[ ]*=
These are all the defined javascript functions you can call
For instance you'll find exports.setText = function(title,field,index,value,options) in the tiddler $:/core/modules/wiki.js
So you can set the text of a field or an index by inserting a link like this:
<a href="javascript:$tw.wiki.setText('tiddlername','text','','The text to set');">Click me</a>
And you click it. It sets the text of tiddlername to that text.
If you set an index on a new tiddler, you also have to set the type in order to make it a JSON tiddler.
In fact most of the good functions are in that wiki.js tiddler
It is confusing that the code says its Wiki but wiki is what works.
Anyway. That's the only one I've used so far. I use it to enable clicking on SVGs.
It would be nice if someone could explain the rest of it.