How to pronounce "TH" with other words?

I have been studying Barron's American Accent. The book says:
"The sound of TH is a special case. It is a floater between areas. The sound is sometimes created by the tongue popping out from between the teeth and other times on the back of the top teeth, combining with various letters to from a new composite sound. For instance, [s] movies forward and the [th] moves back to meet at the mid-point between the two."

I don't quite understand what it means. I always pronounce "th" by the tongue popping out from between the teeth. So what and when is the second case?

More from the book: {when the "th" combination connects with certain sounds, the two sounds blend together to form a composite sound. In the following examples, see how the TH moves back and the L moves forward, to meet in a new middle position.
th+l with lemon
th+n with nachos
th +t both times
th+d with delivery
th+s both sizes
th+z with zeal
th+ch both charges
th+j with juice
n+th in the
Z+th was that
d+th hid those}

Does that mean when say these words together, we pronounce "th" sound on the back of the top teeth instead of between the teeth?

Yes. I tried all those combinations and my tongue stayed behind my teeth most of the time, because the th got reduced quite a bit. "Was that" especially tends to become "was zat", where the th is lost altogether.