These odd bars are there to facilitate the speed-up (that’s accelerando for you lasagna lovers), like how she says “I’m picking it up.” The “bridge” here could be considered a modified chorus. I’m calling it a bridge, though, because of the material it mixes in from all of the sections. This mash-up really is its own thing here, even though it introduces no new material.
No engineer’s library would be complete without some technical information, and Ballou’s Handbook, now in its fifth edition, is a serious contender for being your go-to in this field. A university-standard textbook, it’s not for the layperson.