It's almost harder to write it all down than it is to do it, but I'll try:
We get the IVPB, spike it to a primary line, flush the reseal, prime the line, hang the med, and let it run. When it finishes, we then take the line off the reseal, cap the line, and flush the reseal.
So you wind up with an empty bag of medication that is hanging on a capped, already-primed line. My hospital protocol says we can use that same line for three days, but since we're using the same exact med each time, it's a good thing that the line is primed with med and ready to go.
The next time you hang the med, you just spike the bag, flush the reseal, and hang it. No priming or backpriming is necessary. I've come to appreciate this technique because it keeps our volume-sensitive patients safer, and the reseals tend to hold up just as well either way, so there's really no justification for running a flush bag.
Hopefully that makes sense. Hopefully I'm reading the question right.