Rosalina's DAir can indeed autocancel in a shorthop, but the timing is very strict, requiring the DAir to be inputted practically on the frame the shorthop is done. You basically have to roll your finger from your "jump" button to your "attack" button, both while holding down on the analog. Again, the timing is very tight, but I find provides a surprisingly strong maneuver consisting of both smooth, flowing movement and an attack. With Luma tethered to Rosalina, it becomes downright scary because of how strong we all know Luma's DAir to be. Not only that, but it also provides another way to do autocancelled aerials, allowing for Luma-landings with even the finicky NAir, and it's 1-2 frame autocancel window. On top of all this, also helps with edgeguarding: If Rosalina does a DAir early enough in a shorthop, she can hop over an edge while covering it with DAir, and then recover from the DAir just barely in time to grab the edge she was just previously shorthopping over. I like doing this because it cover the edge twofold, with both an attack in the form of the DAir, and then immediately followed by the threat of a trump, should the opponent have avoided the DAir via edge-grab intangibility. In the event of the former, the DAir threatens with a potential spike if the opponent doesn't receive edge-grab intangibility at the right time. In the event of the latter, the trump still gives Rosalina advantage if they do receive edge-grab intangibility at the right time, but only for you to snatch the edge from them soon afterwards.