In old trannys, or in worn trannys, the synchros may not have much friction or none at all. Then, you need to spin up the input and countershaft manually. To spin them up manually, without being in a gear, you select neutral, then let the clutch out (engage it). If you blip the throttle, it will spin up the input/countershaft and, if you do it well, you will then be able to select the lower gear.
THE ONLY ACCURATE MEANING OF DOUBLE CLUTCHING...........
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The idea of double clutching is to always select netural before a downshift, blip the throttle, then clutch in, then select the lower gear. This is meant to either save your synchros or to be used on trannys that don't have synchros.
The short of it is that you have synchros in your tranny that make double cluthing not required, hence the necessity in Semi truck shifting.
