This was the first ABBA PROMO to be shot on video tape as opposed to being filmed. The main reason was not only to have the desired special effect recreated effectively, but as all ABBA's PROMOs were always mastered on "film" in order to have them screened internationally without technical problems caused by different countries having different video/TV systems.
Information from ABBA Report Issue 11 (Winter 1997) - Alex Jones.
Thanks also to Carl Magnus Palm