As near as I can guess the jiggle pin allows a 5sfe system to cold fill properly by letting air in the bottom rad hose escape past it. Without it, you get an air lock there until the thermostat opens. When it opens the air will pass and the overall coolant level will drop, but if the rad cap is still off, you can then top er up before the rad cap is put on. Once the rad cap is put on, then fill the overflow tank to the hot level.
Just a novice's analysis. Maybe someone else can confirm or adjust this.
85 LE 2SE 500K km - died trying to push a semi off the road
95 LE 5SFE 530K km, 530K km changed engine, 549K km second engine died, now 554K Km running with a 98 5SFE block and head
01 XLE V6 310K km