RB_ROTATE_LEFT (and it symmetric twin) modify the rb-tree, adjusting pointers so that what started as a proper tree ends up a proper tree. When two consecutive rotations move the same node up the tree, some of the pointers changed in the first rotation are immediately changed again in the second - namely, the pointer from the rising node to its new parent, and the pointer from that parent back to the rising node. This change removes from RB_ROTATE macros the responsibility for managing those two pointers, and leaves it to the code that calls for rotations to fix up those pointers afterward. That drops a comparison and a pair of assignments from every INSERT_COLOR or REMOVE_COLOR call that ends in a double rotation.
A side-effect of this change is that the SWAP_CHILD macro must take as a parameter a pointer to the node that is changing children, where it is now computed from the old child. Since this macro is called in a couple of places besides the RB_ROTATE macros, those calls are also affected.