The direct translation of that HTML to Haml is
%button#Save.positive(name='Save' type='submit' tabindex='10')
%img(src='/graphics/icons/save.png')
Save Changes
That button in your example, of course, has a lot of attributes, more than most elements in a typical template. Also, if you are using it with Rails it'd be in a form block with a helper:
= f.submit 'Save Changes', :class => 'positive'
which would generate an <input> that you'd then style with CSS (or Sass :-D)
Note that with Haml you can also write any sort of helpers you want, so you could have something as simple as
= button
doing whatever you want it to.
Where it gets really interesting, however, is when you are dealing with full templates. The document structure is instantly apparent, most superfluous noise is gone, classes and ids are given top billing and templates are very short despite retaining all of the information.