Momentary,,,
Select and hold for ca 1 sec.
I do recall some issues on some sims with regards to this. Some students did a micro second and claimed it did not work.
I then did a quick second ( momentary) and it worked.
It then takes ca 1 min from up to full extend were you have the transit light on until full extend.
I also have done this Live in Hollywood with TE Flap asymmetry frozen at less then 1 and it went from LE Extend to Full extend in less then 30 sec. ( 20? )
With a firm "Momentary" selection of Alt flap switch to activate the stby sys.
Drop the gear ca 8 miles out , get down to Vref +5 asap as there is next to no drag. Ca 3,5 NU 174kts over the fence Autobrake max; ca 1300 meters.
A rather lengthy CL with a lot of crew ending up with a All Flaps Up landing due to misinterpretation or execution.
Good fun in the winter!
Happy landings