Check this link for more info:
http://en.wikipedia.org/wiki/Manual_...ission#Reverse
Reverse is a non-synchro gear, so when you shift into reverse, the output shaft (connected to the wheels) and the input shaft (connected to the clutch) are directly connected by a gear. Basically, the reason it grinds sometimes is because either the car is moving a little or the input shaft is still spinning from when it was connected to the clutch at idle. If the input shaft is spinning, it is stopped abruptly by the reverse gear when it engages, which is when you hear the transmission grind.
So, in practice, the way to avoid grinding gears here is to make sure the input shaft is stopped before shifting into reverse. This can work one of two ways. You can hit the clutch pedal and wait one or two seconds: then the input shaft slows and stops because it is no longer being spun by the idling engine, and the reverse gear can be engaged. Or, shift into first or any other forward gears, then shift into reverse without releasing the clutch. The synchro mechanism of the forward gear will quickly stop the input shaft, and letting you shift into reverse without grinding. It theory, this is the fastest way to shift into reverse.
I have never ground reverse while doing either of these methods, though I have ground reverse when the clutch was out at idle and I quickly hit the clutch and shifted into reverse, or if the car wasn't completely stopped when I tried to shift. Synchromesh transmissions are complicated beasts but worth understanding if you have the time and patience. I haven't figured it all out exactly, but I checked wikipedia and I'm
pretty sure this is right.
Since the gears still have to mesh, the teeth can hit each other instead of interlocking, which prevents you from shifting, that wall you were talking about. In my experience, popping the shifter back into neutral and trying again usually works. My guess is that the input shaft would rotate slightly from moving the shifter around, so that if the gear teeth don't mesh at first, they will on the second or third try as the transmission is jiggled around.