Yes, on most models of the Braun Multiquick 5 you have to keep the control button (or the turbo button) pressed as long as you want to use the blender. As soon as you release the button, the appliance stops.
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Why is this so? - Safety function: So that the blender only runs when you deliberately press it. - No continuous operation function (no engaging): The Multiquick 5 is designed for short operations (e.g. pureeing, chopping).