In order to use the alternate gear extension you are supposed to have the main lever in OFF position and move it to DOWN as a final step of this procedure. I wonder what would happen if it was in UP or DOWN position while putting the alternate switch to DOWN. Is it possible to lock some hydraulic valves in wrong position and disable the entire system, main or alternate, by f.e. leaving some hydraulic pressure?
In short, yes it is. To use ALTN Gear extension, the Landing Gear Lever should be in the OFF postion (hydraulics in a "runaround" type circuit) so if there is any hydraulic pressure available, gravity extension will merely push the fluid around the circuit. If the lever is left in the UP postion then its possible for fluid/pressure to be trapped in up circuit and so resist gravity extension.
The lever is placed to down after extension to satisfy indication and configuartion warning circuits (as well as GPWS alerts). Possibly even antiskid operation but not entirely sure of that as I can't remember.