IIS Url Rewrite under Joomla and SH404sef

I am using a joomla3 website with sh404sef, under IIS

so far so good, sh404 works correctly, SEF urls are correctly handled

I am now trying to add a custom redirect rule in web.config

basically I want to rewrite this kind of URLs:

www.mysite.com/myfolder/mypage.html

to:

www.mysite.com/mypage.html

for each page under that folder...

this is the rule I wrote:

<rule name="assets redirection" stopProcessing="false"><match url="^(.*)/(myfolder)/(.*)" ignoreCase="false" /><action type="Redirect"    url="www.mysite.com/{R:3}" /></rule>

but he rule seeems to be wonderfully IGNORED... if I test the pattern in IIS site manager it is applied correcty, but no action seems to be taken...

Wy is it ignoring the rule? could it be a sh404 or IIS related problem?