The one they all have is used to keep the TB warm by passing from the steam pipe thru the TB & back to the overflow.
The bypass takes it from the steam pipe running up the pass side (between valve cover & intake) & sending it directly into the overflow.
I think the idea is to allow any air to stay up high in the circuit
That allows the air pressure (expansion) to be delivered on top of the fluid
It doesnt pump down because the systems full.
If the cars allowed to get hot (without a capped tank) the fluid will rise & overflow (if overfilled)
I think thats what I know
96BBB 28K 3.42 Bolt-ons
95BBB 100K 3.73 383 New DD...already
fixt the 4L60E twice
95 Black WB4 Hole...ee...grail
96 Black WB4 No holes in this
95BBB 189K 3.08 Old DD