i guess its when you can feel when its going to grind and press the clutch down again before it does to catch it.. i've recently become quite good at this seen as 2nd to 3rd gear can be a bit temperamental on my car. I can feel it when the gear lever doesn't quite slot all the the way into 3rd so just as im lifting the clutch pedal i quickly press it down again to save the grind! Such a horrible noise, feels like something breaks when it happens