slip·stream 
/ˈslɪpˌstri:m/ noun
plural slip·streams /ˈslɪpˌstri:m/ noun [count] technical : an area of low air pressure that is immediately behind a vehicle that is moving very fast and that other vehicles can ride in to go faster with less effort
▪ The motorcycle was riding the truck's slipstream.
▪ The motorcycle was riding the truck's slipstream.
