I'm speculating, but here goes: "Send it" has been in use in sports for awhile, and the usage is literal: cause the thing to go, and from that point it's betwe

I'm speculating, but here goes: "Send it" has been in use in sports for awhile, and the usage is literal: cause the thing to go, and from that point it's between physics and whatever god(s) you pray to. Typically used for the act of passing/shooting. "Full <thing> mode" has also been in use for awhile, meaning an extreme or complete version of a tactic or state of being. Combine the two, and you get "full send mode", where you make a move that completely removes your control from that point forward. Then you drop the last word for brevity, and you have people going full send.