Unless you are a fully loaded 18 wheeler, use your brakes to slow down, not the engine and trans.
Hopelessly incorrect & terrible advice. Fail.
See Fobroader's comment above.
+1 (sort of)
Yes, downshifting and dumping the clutch is a bad way to slow the car down: that's what brakes are for.
But, there is nothing wrong with executing a proper rev-matched downshift to put your car in a lower gear, then using that lower gear and engine braking to
maintain your speed on a downhill.
Even with an automatic, engine braking is your friend. Just make sure to downshift into the appropriate gear at the
top of the hill, not half way down when you're already going too fast and want to slow down. Again, it's all about using the gears to help maintain a chosen speed, not scrub it off once it's too late.