During tight turns the "twist" or washout designed into the wing by Reginald Mitchell meant that the wing root would stall before the wingtips, creating the shuddering and clattering referred to. This noise was a form of stall warning, reminding the pilot to ease up on the turn. British testing in September 1940 revealed that some Bf 109 pilots succeeded in keeping on the tail of the Spitfire, despite the latter aircraft's superior turning performance, because a number of the Spitfire pilots failed to tighten up the turn sufficiently. The gentle stall and good control under "g" of the Bf 109 were of some importance, as they enabled the "Luftwaffe"' pilot to get the most out of the aircraft in a circling dog-fight by flying very near the stall. The Bf 109 used leading edge slats which automatically deployed prior to stalling, but also made it much more difficult to continue chasing either a Hurricane or Spitfire with a tight turn in aerial combat manoeuvres, from the slats intermittently opening in tight turns (on the wing to the "inside" of a turn) during dogfights, even causing problems during a takeoff if they opened unevenly.