What is Stream Sniping?

Solution 1:

Stream sniping means watching a player's stream with the intention of queueing for a game at the same time as them in order to be matched with them, either on their side or against them.

In the case of the video linked in the question, the sniper is trying to join the streamer's team in order to "throw", or lose the game on purpose, with the objective of causing distress to the streamer.

Over time the meaning has changed to include gaining an advantage against a player by watching their stream, but the correct term for that is "ghosting", not stream sniping. Gaining such an advantage in faster-paced games like Overwatch is also harder, which makes the "ghosting" meaning of the word less likely, plus the fact that the sniper is referred to as a "thrower", which means they're not trying to gain any advantage for themselves, and in fact want to lose the game.

The term "ghosting" predates streaming, and gets its name from many first person shooter games in which dead players can spectate their living teammates from a third-person perspective with a certain degree of freedom. Doing this they can, for example, peek around a corner or into a doorway and warn their teammates about any traps or danger on the other side.

On the other hand, "stream sniping" gets its name from the long time a sniper has to wait while the streamer is already in a match, and the short window of time during which they should queue if they want to be matched with the streamer. This tactic -waiting a long time for a short window of opportunity- is reminiscent of sniping.

The evolution of "stream sniping" from "joining a streamer's game" to "gaining an advantage against a streamer" comes almost naturally when slower-paced games are involved, as the sniper/ghoster has time to assess the streamer's situation more calmly, without being distracted from their own gameplay.

I'm unable to find any concrete "sources" on these terms, as is often the case with gaming-related terms, but Wikipedia uses "ghosting" to refer to the act of gaining an advantage from a perspective a player shouldn't have access to, while also mentioning the usage of "stream sniping" as a synonym. TotalBiscuit also seems to believe in the difference between these two terms, and has a vast knowledge of the competitive side of StarCraft 2, a game where ghosting can provide a significant advantage.

Solution 2:

Stream sniping is when a player is in a game against an opponent who livestreams their game online (on a service like Twitch), watches their stream on a second screen while playing and uses the knowledge from the stream against the streamer.

When a streamer gets killed in a way which looks suspicious, then it is often hard to tell if their opponent was actually watching their stream or was just lucky. This especially applies when the streamer doesn't have a large number of viewers, the game is a very popular one and players can not influence who they get paired up with. So you shouldn't take every allegation of stream sniping at face value.