That thing sets the speed level of the intermittent feature of the wiper. You set the lever at the first level for the intermittent feature then you fine tune the speed you want the wiper to wipe intermittently by turning that dial.
^ also to add, if you have rain sensor wipers, put it all the way down on the bottom if you want the rain sensing to work. otherwise it will intermittently wipe at whatever speed you chose on the lever/dial
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