I know it can be avoided and teched, but if it is not it is a spike. So just for information purposes it works on everybody except ROB after 76%. Not sure why ROB alone, but even at 200% it does not work on ROB. Some characters do weird things when thrown, some hit the underside of the platform and go down, some hit the side of the platform and go down and back a little.
Some people can recover from the underside spike, in that case you can do the side spike which gives you enough time to run off and do a dair and spike them again.
Metaknight is weird, he will hit the side of the platform and hit the side of the center platform then go in the water. If spaced correctly he will hit the side of the platform and fall back on the center platform, he will just be lying there. Easy sakurai combo, grab to sakurai combo too good.
Others characters you have to walk off and throw them for it to be a spike, can be risky due to claptraps.
Also there are plenty of mixups, instead of doing the down throw, you can walk off and do the up throw, and throw them up into the bottom of the platform. They will have DIed the wrong way since they were expecting the normal down throw. Also turn around as you are about to fall off and throw them under the center platform. Doing any of these will cause you to go in the water as well, when you recover from the water with your up b you will hit them as they pass you and stun them making it harder for them to recover.
All of that can be done if it is not teched or DIed, if you try for the underside spike and they tech it do a rar bair and then upb. You will hit them if they are not fast enough to get out of the way, the bair will stun them, the upb will hit and knock them up into the platform spiking them in the water. If done correctly Dk will not fall in the water, if they tech that hit as well, then I say they deserve to live.
Many other things can come from this, I will try to make a video of all the things that I have found.
The character have to be at their respective percentages in order for any of this to work, it can start for some light characters at 58% while it will not work on others such as Bowser till 73%.