True, the "hand-waving" thing has somewhat become a symbolic gesture to indicate a strike. Obviously in training we're not out to hurt each other, but tori should... um... "gesticulate"... with the intent to land the strike, should uke not move, and likewise, uke should react appropriately as if it were a strike...
Sometimes these gesticulatings get lost during the waza. I remember (years ago when I was 5th kyu) doing a fist strike to uke's face with the intent that he would either raise his hand to block/incept or lean backwards. The guy didn't do anything that I intended him to do and I had to stop my punch barely a centimeter from his nose. Anyone less experienced wouldn't have control that punch and he would have landed up with a bloody or broken nose. When I asked him why he didn't move, he answered sarcastically that he trusted me not to hit him.
At another time when I was an uke instructed to attack with a mae geri. The nage was a yudansha, knowing that he didn't have experience in percussion art, I kicked with 60~70 speed. I expected him to move moment before the contact but he didn't even move a bit. Despite pulling back, my kick still touched him at his balls. He screamed at me for kicking him and I told him that it wasn't my fault - he was supposed to move.
I am still "confused" with this hand waving gesture especially at some dojo that practice this. The nage would waved their hands about my face and just when I raised my hand to block, they would drop the hands down as fast as they came up leaving the openings for me to strike back with my raised hand (of course I wouldn't do that). I realized where these were coming when I took ukeme for the dojo-cho. Times when I thought I didn't need to raise my hand in response to their gestures, I got smacked on my face.
I could figure out why they do these at some dojo and but not all the time. I will explain this at a later time and probably talk about an atemi which in Shorinji Kempo called "Me Uchi".