My father instructed in AT-6 (T-6, Texan/Harvard...) aircraft during WWII - and at one point their curriculum including partially blanking off the ASI window so that the student could not read anything below 100 mph (90 kt). The objective obviously was to give the student confidence in tactile and attitude airmanship (airwomanship actually; his students were WASPS). Although this practice didn't last too long, Dad regarded it as a good exercise.