This works only with Minecart Experiments enabled.

When a piston pushes a minecart into a block, the minecart does not rotate. When the piston pushes multiple minecarts (a minecart cluster) into the same block, some or all minecarts rotate, depending on the number of minecarts. This can be seen in the attached video.

Steps to reproduce:

1. Build the setup as shown in the video.
2. Place around 10 minecarts on the rail.
3. Break the rail.
4. Activate the lever.
5. Deactivate the lever.
6. The minecart cluster rotates.