This is the default behaviour, normally. But in some 
cases (redirects) this might not be.
In this case, or if you just want index.htm AND ONLY 
dirs BELOW, use:

-* +intranet/products/product1/index.htm 
+intranet/products/product1/*/*

That is, forbide everything, then accept the index, 
and files BELOW (and only below, not in dir)