Here is why you double clutch:
When you are in a higher gear, and want to downshift, i.e. 3rd to 2nd, 2nd to 1st, and are moving, you double clutch.
Lets say you wanna get into first while moving, and your in second: You press the clutch, and place the shifter in nuetral. You release the clutch, and rev to the RPM the engine will be at when you finally release the clutch. Then, press the clutch, slide the shifter into first, and release the clutch quickly- then floor it. You've rev-matched the speed of your gears to the speed the car is moving, so the load is not all on the syncro. This is ESPECIALLY important if you want to downshift to first. NOT doing this will result in a severly worn out syncro in a short period of time if you downshift without double clutching, therefore resulting in grinds.
See, if you dont double clutch, your syncro has to speed first gear up to the speed of the car. That "block", or resistance you feel trying to downshift to first while moving is not actually a block, it is your syncro attempting to speed up the gear. See, if you held it gently against it (not reccomended cuz it will severly wear your syncro), it will go in, even at 60mph (DONT LET OUT THE CLUTCH)!!
I hope that clears things up...