The first thing out of the line is not your concrete
If you are standing on your own pour when the pump starts, the first material out the end will not look like concrete at all. It comes out as gray soup. That is the priming slurry, and its job has nothing to do with your slab. It coats the inside wall of every section of hose so the concrete behind it slides along that wall instead of grabbing at bare rubber and steel.
So it goes into a barrow, a tub, or onto the ground at the spot picked for washing out, and it does not go into your forms. Anyone who tells you to tip that first material into a corner of the slab because it is only a little is not doing you a favor. It is cement and water without the aggregate that makes concrete concrete, and it belongs out of the pour.
Priming happens before the concrete, every time
Nothing goes down a dry line. The order on pour day is fixed. Park the trailer where the mixer can reach the hopper. Run the hose out along the route that was walked beforehand. Prime the line. Then start taking concrete off the truck. All of that finishes before the first yard is placed, never during it, and that is why we plan to be set up before the first truck backs in rather than while it waits.
The reason it never gets skipped is arithmetic that goes badly. Priming takes a few minutes. Finding and clearing a packed section takes a good deal longer, and it happens with concrete already in your forms and more of it on the road. Trading the first for a chance at the second has never once been a good trade.
Where it ends up, and why that is decided early
The slurry has to land somewhere, and so does what comes out of the line at the other end of the day when the hose is cleared and the hopper is washed. Those are the same decision, and it gets made before anything starts rather than at the end of a long morning with everybody tired.
The spot wants to sit away from anything finished, nowhere near a storm drain, and in a place where hardened material in the dirt bothers nobody. On a rural lot there is usually an obvious corner for it. On a tight lot in town it needs more thinking about, and that is exactly why it gets picked while we are walking the site with you, rather than while somebody stands there holding a running hose.