I agree with Randy, I have installed bridging in floor framing since the early 60’s, and it is required to provide a complete diaphragm for proper load support. In the post above, the builder forgot or was to lazy to crawl in and finish the job. It may have lasted this long, only because it was not required to carry the floor load if the framing was the right size for the span. It just went on to provide a floor that was bouncy and never had a concentrated load to accentuate a problem to the framing members.
it was nailed in before the sub flooring and when the sub floor was complete, you went underneath and finished nailing it.
It was not meant for rotation, but more for completing the diaphragm to make the floor system more stable and less bounce.
Any load imposed on the floor system is distributed equally rather than on just the immediate area.
I found this article that says the same thing.
Floor plans or specifications usually call for bridging between joists. Bridging holds the joists in line and helps distribute the load carried by the floor unit. It is usually required when the joist spans are more than 8 feet. Joists spanning between 8 and 15 feet need one row of bridging at the center of the span. For longer spans, two rows of bridging spaced 6 feet apart are required. **CROSS BRIDGING.— **Also known as herringbone bridging, cross bridging usually consists of 1- by 3-inch or 2- by 3-inch wood. It is installed as shown in figure 1-26. Cross bridging is toenailed at each Figure 1-26.—Wood cross bridging. end with 6d or 8d nails. Pieces are usually precut on a radial-arm saw. Nails are started at each end before the cross bridging is placed between the joists. The usual procedure is to fasten only the top end of the cross bridging. The nails at the bottom end are not driven in until the subfloor has been placed. Otherwise the joist could be pushed out of line when the bridging is nailed in.
No one is going to convince me that it is only required for higher depth joist.
Higher depth joist like bar joist, yes, rotation under load comes into play and the bridging is playing a double role.
Standard home floor framing is a different application. Nothing is going to rotate, it will bend and fail.
Higher web members in commercial or engineered floor joist, will rotate and fail.