This is desirable because the aircraft's flight control surfaces are often located at the wingtip, and the variable stall characteristics of a twisted wing alert the pilot to the advancing stall while still allowing the control surfaces to remain effective, meaning the pilot can usually prevent the aircraft from stalling fully before control is completely lost.
Twist that decreases the local chord's incidence from root to tip is sometimes referred to as washout.
Twist that increases the local incidence from root to tip is less common and is called wash-in.
The Grumman X-29 had strong wash-in to compensate for the additional root-first stalling promoted by the forward sweep.
[citation needed] Wing twist can also, rarely, refer to the deflection of the wing when it is made of insufficiently stiff materials; actuation of the flaps can, instead of deflecting air as intended, cause the wing itself to be deflected and is related to compressibility effects; this problem has mostly been eradicated however, with modern high-strength alloys and composites.