HI,
There two kinds of Wing Anti Ice systems installed in classics 737's.
One that positioning the WTAI switch ON opens both control valves if thrust on both engines es below the setting for take off warning activation and the temperature inside ducts is less than the thermal switch activation temperature. On ground with WTAI ON , the switch remain in this position regardless of control valve position, when the air/ground sensor goes to the air mode, the switch automatically trips OFF. In flight this logic is desabled.
The other system, based on the air/ground sensor prevents the wing anti-ice control valves from openning on the ground excpt during ground test. As a procedure i.e. low altitude ice conditions exist or are anticipated, the switch is placed in ON on the ground, the vavles are closed, but the system is armed for flight.
During liftoff air/ground sensor enables the WTAI system and both valves open. A thrust penalty is taken du to WTAI bleed air extraction.