I instigate a Level-4 Unidad alert, then I find an elevated position with the most possible LOSs, get on top of it, and do the Macarena.
LOL, no. With the exception of unique poses for photo mode, they're almost exclusively multiplayer (GW, Co-op) phenomena. Typically, they're executed in GW in the interim between having eliminated the opposing force and recapturing an objective (ie. broadcast radio) for the purpose of taunting.
hahahaha i get killed because i was behind a wall and i sign my friend (cover me) but nomad instead to do the sign from cover i just stand, so i get killed, sometimes i wonder who design this things, but as V4MPiR1 said just for show or photomode otherwise you have no idea what the other player is meaning