Wattup,
When editing the text field of any tiddler you can use wikitext which is a "mark up language" search for wikitext at tiddlywiki.com
The editor Toolbar also offers most of the wiki text as buttons - just select some text and hit B to bold it, at least until you learn.
However you can get additional features if you install the codemirror plugin that improve the way the editor works especially if you are a programmer.
It is also possible to install the visual editor and it looks much like any WISYWIG editor but the result is stored as HTML which is not always as helpful if you want to use macros etc...
By learning the wikitext markup, you learn to write quickly using a few simple characters to change you text into headings and bullets etc... then you are set up to do a lot more with your tiddler content.
It you are editing a tiddler click on the editor toolbar button, click the "closed eye" icon to open the eye and display the preview right along side.
So yes you can "sort of 'switch' from source code to real time preview mode? we tend to call it edit and Preview
Regards
Tony