No electric arc is struck because the current flows through the arcing contacts. During its run downwards, the moving apparatus compresses the gas in the lower chamber. The compressed gas flows from the lower to the upper chamber. hereby taking both the chambers to the same pressure.
Arcing Contact Separation:
The current flows due to the electric arc which has struck between the arcing contacts. The gas cannot escape through the nozzle because the hole is still closed by the fixed arcing contact and cannot even escape through the inside of the arcing contact, because of the clogging effect of the electric arc.