At a given IP compressor pressure ratio, opening the blow-off valve forces the IP compressor entry corrected flow to increase, to a point where the IP compressor surge margin tends to be better. Effectively, opening the blow-off valve lowers the IP compressor working line. Any flow surplus to that demanded by the HP compressor passes through the blow-off valve into the bypass duct. The blow-off valve is normally only opened at throttled conditions, since it wastes energy.