I think this is the bit that some fans (aka, the "get it in the box" brigade ) don't quite appreciate...
Drawing the opponent out
Southampton's attacking play depends on being able to go up the pitch with momentum so that they catch teams in transition (which in theory is when they are most vulnerable), something they can't do when the defending team sets up in an organised block. One particularly devious ploy that they use to avoid this is passing backwards from advanced attacking areas, even when there's an opportunity to cross or get closer to goal.
We saw it a few times against Liverpool, with Kyle Walker-Peters on the ball, in the example below, forming a front four. Danny Ings is the main attacking threat from crosses but is left wing, so the chances of scoring from a cross played into the area are low. Walker-Peters takes a beat.
In slowing play down, Liverpool have a chance to get back into their defensive shape and the chance to score is gone. Walker-Peters then passes back towards his defence.
Liverpool then push forward to follow the ball and try to win it back close to half way, but Southampton are in position to recycle the ball in their little triangles.
James Ward-Prowse passes (1) back to Jan Bednarek, who passes (2) to Ibrahima Diallo, who has three available options for the next pass. Behind Diallo, Liverpool's defensive line is squeezing up. One smart pass over the top could spring the offside trap and play Southampton's pacy forward line in behind but on this occasion, Diallo is fouled. Southampton don't simply lump the ball into the box, nor do they patiently probe in the final third. They are built to harrass, hassle, wait... and spring a trap