The Kindle uses the toc.ncx file for this section to section movement.
You can unzip an epub file to see what one looks like and edit it for your own use.
Then reference the ncx in the mobi .OPF file's manifest and spine sections:
<manifest>
<item href="toc.ncx" id="ncx" media-type="application/x-dtbncx+xml"/>
.
.
.
</manifest>
<spine toc="ncx">
.
.
.
</spine>
To create sublevels in the toc.ncx, write something like this (note the placement of the opening and closing tags):
<navPoint id="book-1" playOrder="1">
<navLabel>
</navLabel>
<content src="filename1.html"/>
<navPoint id="sub-chapter-1A" playOrder="2">
<content src="filename2.html"/>
</navPoint>
<navPoint id="sub-chapter-1B" playOrder="3">
<content src="filename3.html"/>
</navPoint>
</navPoint>