Whilst sliding on a rail, one swings his feet in opposite ways making them briefly staggered. like a daffy, but on a rail. Once in that brief state, on grabs juts in front of the binding on the front ski, which is staggered forward and just behind the binding on the back ski, which is staggered back.
The name comes from the trick Daffy, which is also the name of the cartoon character, Daffy Duck. There is another cartoon duck, named Donald Duck, hence the name Donald. And like the trick a TRUCKERslide, in which one grabs a TRUCKDRIVER grab while sliding a rail, a Donaldslide slide is a DAFFY GRAB while sliding a rail. It is a rail trick invented by yours truly. I don;t have any vids of it right now, but i will definitely have one up on the interwebz soon enough.