I think from APA folk you will commonly get both legal and not-legal responses. I think you will get that variance from people who play pool in general, as you see in this thread, because the rules tend to be ambiguous.
As Scott and another called out, it's part intent, and part forward motion of the cue that makes the shot legal... Sort of... But even forward motion in APA, BCA, WPA, and otherwise is not always the test so at some point it becomes ambiguous.
Forward motion is not always the case because one can make a legal shot by aiming and stroking straight down on the cue ball to make it masse... Some we see once in play a while and is not called a foul because we see the person stroke the cue, propel the ball, and do so without double contact. No forward stroke required, completely legal.
To me this is a similar move to scooping, except one is not actually lifting the ball off the table or digging into the cloth. In APA scooping is clearly illegal,but this is more ambiguous because it's not explicitly called out in the rules. I am fairly sure my LO would say it's illegal if he saw it happen, but I'm certain I can find other opinions from other LO's I know. I feel like it should be explicitly illegal because it just ain't natural and it's un-American dammit!