I'm sure Jo or Dannie will provide the professional answer shortly, but from a home groomer here's my interpretation, yes you should cut in the same direction as hair growth but if you hold the scissors so they push into the coat slightly (as oppose to resting on it if you see the difference) then you will cut hair. I work on the theory with thinners that I'm after taking off a little bit, have a look at how it looks, take off a little more, have another look etc. It can be hard to imagine sat in front of a computer, but get the dog on a table, hold the thinners how you would normally and now just change their direction so they point the same way as the hair and you'll quickly see that they do cut. As an example, if you were thinning hair on the dogs back the point of the thinners would be aiming at the tail, the finger holes at the head end, following the spine and the hair growth.
One tip from experience; do NOT cut a line 90 degrees to the direction of hair growth (for the example given above don't cut in the same direction as the ribs!), you get a stupid dotted line through the coat that takes a fair bit of work to make normal looking!
Cara and Mo, two black mini's served by Elaine, the cook, poop picker, and chief toy thrower.
I live by the Mini Schnauzer code; when in danger or in doubt, run in circles, scream and shout!