Incremental Search with marked text

[Andrew C. Suttles]

If I mark a word by double clicking (yuck) the mouse, I can do an
incremental search for the word by using c-s m-y.  This 'yanks' the marked
word into the minibuffer as the target for the isearch.  However, when I
use m-@ to "mark word" and then try the same thing (c-s m-y) I get the
previously marked word and not the current one.  

What commands are invoked by a double click.  I tried using the
help-describe function, but it will only describe the first click, not a
double click.

Thanks