Depending on the contractor and the condition of the existing pool there are multiple way to prep the surface. A chip out removes most of the existing plaster, often used when the existing plaster has de-bonded ( is hollow) from the gunite or has been replastered multiple times. The plaster is mostly chipped out exposing the original gunite, and the new plaster is applied over the exposed gunite surface. Water jetting and sandblasting are typically used when the plaster is soft with little or no debonding and the contractor feels that a chip out is not needed. Both of these methods remove the soft plaster, in some cases down to the gunite, and a bond coat is typically applied. Acid washing and scarring the plaster are used when the existing plaster surface is not soft or debonded. The acid wash or scarring roughens the surface and a bond coat is applied. Sandblasted, water jetting, acid washing and scarring all involve chipping around lights. pipes, and under the tile so the new surface finishes flush. I think that the best method is the one that properly prepares the pool for replaster, with as little damage to the gunite as possible. The key is "properly prepares the pool".