I now have more information about this spike. First off, of all the spikes that do hit, the all go to about the bottom of the crystal thing on final destination. Also, if the knee trips the opponent they will either grab the edge or just trip on the ground depending on how close to the edge you are. Now I will talk about the effect of this on every character. First the two characters that this instantly kills: Ike and Link. For this to happen you simply hit them with the knee part of Snakes f tilt and if done correctly it will spike them, then fall off the ledge do a b air and recover. Link will be too far for his up b or tether to reach and he will die. Ike will be too far down for his up b to reach and he will die. This will work for link until 64%. up until then it will slowly start hitting horizontal. This works for Ike until 999% however. Now I would also like to note that snakes jab, the first hit, semi spikes. Now on to the characters that can be instant killed but its a bit trickier. Ice climbers can be spiked because for some reason you only hit popo off and nana stays. He will fall off and not be able to use his up b. Then theres wolf. Wolf can be spiked but you must hit him against the ledge(the part that you grab) then his side b or up b will not reach. Last there is toon link. His is just like links but its a bit more difficult to land the b air just right. Now for the rest of the characters that it either doesnt work on or it does work but its not an instant kill. It does not work at all on samus, she will grab the edge within a couple seconds. It will work on mario but you hav to do an extra thing for him to die. first you do the spike, then you wait till he jumps back up, then you use d tilt to f tilt and he cannot make it back. For marios %s, slowly turns into semi spike until 24% it then turns into a horizontal hit. Fox until 999% but his up b will always allow him to come back. pikachu works until 999% but she can get back with up b. Marth is wierd I tried several times but I always get a different result. Sometimes it works and sometimes it will be a semi spike but it works until 999%. Mr Game and Watch works the exact same as marth. Luigi is always a small semi spike as is Diddy Kong. Zelda sometimes, it will either be a small semi spike or she will grab the edge. Sheik works to 999% and falls farther than any other character. I sometimes got her but its tricky because of her up b. Pit yes but small semi spike. The jab causes a better semi spike for pit than the knee. Metaknight is the same as pit. Falco yes but jab works way better you wont knock him off because when you use b air he can just use side b unless its high percents. It works on Ivysaur but he will grab the edge with a couple seconds or jump back on the platform so its pretty much useless against him. Squirtle yes but jab works better. Squirtles up b gets him back. charizard jumps back too quick for you to do anything. snake yes but up b gets him back. peach jumps right back. Yoshi will most likely do his double jump but you cant kill him because of the super armor it has. ganons up b gets him back even after b air. DeDeDe almost always will get hit against the ledge if you attack right away but his up b always get him back. It doesnt work for lucario he will instantly grab the edge. Ness and Lucas can only get back if they are good with pk rocket and lucas will most likely get back since his up b is easier to control. Sonic no because recovery. Bowser no because recovery. Wario no because insane recovery. Rob will instantly grab ledge. Olimar no because he will always get back even without any pikmin amazingly. Captain falcon will just get up with up b. jigglypuff will not work. I hope this was somewhat helpful. I know it may be hard to read but I cant do alot of fancy stuff because I am using my wii internet. I hope this technique will be useful. It obviously will be most useful for the first few listed characters.
Maybe space every other character you are describing this spike for.
