In the standard _main.php file, the content of the articles is shown like this

<?php $item->content() ?>

when using nothing between the brackets, it is using the default values,

It will show the article till the 'read more' text and on the read more text, place a link to the complete article, and it will show that in the normal template.

to be complete :

this :

<?php $item->content() ?>

is the same as this :

<?php $item->content( '#' , '#' , '#' ,'#' ,'#' ,'#','htmlbody' , 0  ,false ,'' ) ?>

these are all the parameters :
Parameters:
1 $disppage: page number to display specific page, # for url parameter
2 $dispmore: true to display 'more' text, false not to display, # for url parameter
3 $more_link_text: text to display as the more link , # for default value
4 $more_anchor: text to display as the more anchor (once the more link has been clicked) , # for default value
5 $before_more: string to display before more link/anchor , # for default 'nothing'
6 $after_more: string to display after more link/anchor , # for default 'nothing'
7 $format: Output format, see format_to_output()
8 $cut: max number of words
9 $stripteaser: true if you don't want to repeat teaser after more link was pressed
10 $more_file: filename to use to display more, '' is default, and that means the same file

So, if you want something else to happen, play with these parameters.