TriAya is correct, descend means to go faster. Specifically for your workout you will want to swim the LAST 400 (of the 800) faster then the first 400:
"Swim: Strength/Speed WU: 600 MS: 4x200 on 20" rest 800 swim, descend at 400. Rest 30" 8x100@ 90% on 10" rest CD: 200"