roys dtilt, along with every other move, has an FAF (first acitonable frame), or IASA (interruptable as soon as). they are the same thing. this is whats being talked about when you here of interruptable frames, since moves performed on the FAF typically "interrupt" the ending animation, which exists solely for animation purposes, and you can act out of it.
as is, roys dtilt is active on frames 8,9, and 10, and FAF is on frame 20. however, this is a "selective" FAF in that you cannot shield out of it. you can do any action except shield on frame 20, and you have to weight the full duration of the ending animation (56 frames) in order to shield. however, you can easily get around this by tilting forward while attempting to shield at the FAF. This technically has you walk for one frame, then shield. note, most moves arent like this.