i have no idea how to do it, it just happens to me randomly like when i Fsmash, and popo does it but Nana dash attacks. or sometimes when i grab nana does a moonwalk dash attack(pretty weird). but yeah, can anyone enlighten us with information
Well, the first one is a known desynch. If you dash in the opposite direction you face (I don't know if it can be done forwards), you have a six-frame window to hit A. In that window Popo will have started to dash but Nana will still be stationary, and it will be a close enough call to make her f-smash instead.
The second one it just really weird. I've had that happen to me too. My guess is Nana was facing away and sliding towards you faster than her walking speed, and the dash attack was done in a close enough range that she still moonwalks towards you.
As for the original post, it would be really cool if we could actually find out how to control this. Throwing Nana into a body tackle is probably even more helpful than a flying blizzard. Or, it could only be Nana acting weird. I've had her u-smash a recovering opponent after I ledge-hopped a (missed) b-air, killing them. I didn't even do anything.