This make sense because the lattice parameters given were for NaCl, not MgO, so we expect them to be too large at the start, and VASP will relax to the lowest energy lattice parameters. Notice that the pressure starts and ends slightly expanded. Beware: In the second to last iteration the volume of the cell drops to 5. This is about 10 times greater than the pressures at the center of the Earth! Something weird happened in the relaxation here.
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The method works by optimizing a number of intermediate images along the reaction path. Each image finds the lowest energy possible while maintaining equal spacing to neighboring images. This constrained optimization is done by adding spring forces along the band between images and by projecting out the component of the force due to the potential perpendicular to the band. A climbing image method  and a better tangent definition  combine to allow for the more accurate finding of saddle points using the NEB with fewer images than the original method.
The setup and operation of this implementation can be identical to what is described in the vasp manual under the elastic band section.
This image does not feel the spring forces along the band. Instead, the true force at this image along the tangent is inverted. When this image converges, it will be at the exact saddle point. Because the highest image is moved to the saddle point and it does not feel the spring forces, the spacing of images on either side of this image will be different.
It can be important to do some minimization with the regular NEB method before this flag is turned on, both to have a good estimate of the reaction co-ordinate around the saddle point, and so that the highest image is close to the saddle point.
If the maximum image is initially very far from the saddle point, and the climbing image was used from the outset, the path would develop very different spacing on either side of the saddle point. The system is an Al adatom on an Al surface. The process is an exchange between the adatom and a substraight atom, leading to adatom diffusion. Notice how the climbing image calculation has shifted the position of the images by compressing the images on the left so that one image sits right at the saddle point.
The cNEB energies have been shifted by 0.
How to perform phonon dispersion calculations using VASP?
ASP and ASP.NET Tutorials
The VASP Manual