In short TextExpander automatically replaces certain letter combinations with other text snippets. If, for instance, I write "xbr" it is replaced with "Best regards, Niklas". This saves me a lot of time since I have many snippets that I use several times every day.
To save time on the iPad or iPhone I don't want to have to use shifted characters more than absolutely necessary. Therefore my goal, when writing markdown, is to have text snippets for all markdown that contains shifted characters. In markdown, # at the beginning of a row creates a big heading. To write # I instead write "xho". X for "expand", h for "heading" and o for "one". To write ##, for the second biggest heading, I write "xhtw" = expand heading two.
Here is a list of my most used markdown abbreviations for TextExpander Touch:
|xho||#||heading size 1 (one)|
|xhtw||##||heading size 2 (two)|
|xhth||###||heading size 3 (three)|
|xhref||[linked text](http://example.com/ "Title")||HTML hyper link|
|ximg||![Alt text](img.jpg "Optional title")||image|
|xas||*||used for lists, bold and italics (asterisc)|
Feel free to improve on this list.