A weft yarn deflection brake including a braking element 5 which can be adjusted in timed fashion and by its variable braking force, a drive motor 6 for said braking element and a control device CU connected with said drive motor 6 is equipped by a position detection means E associated to said braking element 5. Said position detecting means E is connected to an adjustment means 9 for functional parameters of the deflection brake. During operation functional parameters as e.g. the braking force and the time of the activation of the deflection brake are varied adaptively in order to assure a performance of the deflection brake which is deemed to be optimal for the insertion system. At selected points in time known target positions of the braking element are set and compared with the actual positions. In case that deviations are detected a respective functional parameter is varied.