kid craft 24/sonic master
Smash Lord
The dair spike is within a 4 frame window after the inital 2 frames of the attack being active. Basically dair starts hitting on frame 17 and 18 with the hotbox being that strong vertical trajectory. Frames 19 to 22 are where you can spike and anything after that is the sourspot. So to explain it simpler just try to spike someone shortly after sonic starts descending downwards to land the spike.