What Rebirth changes
Rebirth clears your level and Chakra and gives a higher Chakra multiplier for the next run. You can rebirth once you reach the level the Rebirth panel asks for. That level is not a single published number.
One walkthrough says Wins are kept. Retention of eyes, pets, eggs, Trophies, stages, and auras is not on a published table. Read the confirmation text in your session before you accept.
When to press it
Reset when the current stage has stalled and you already bought the eye or aura you can afford. Do not reset to fix an unequipped eye, an unopened code egg, or a skill bar you have not read.
After the reset, train on a pad you can use, equip the best eye and pet that are still available, and climb back. If the same wall returns and the rebuild was not faster, change the loadout before you rebirth again.
Auto Rebirth
Auto Rebirth is a setting you can turn on and adjust. Its unlock level, trigger, and what it saves are not published. Watch one automatic reset before you leave it running. Turn it off when you still need to spend a resource the reset might clear.
Training areas in later rebirth counts exist. A walkthrough mentions multiplier pads and areas tied to rebirth milestones, including a jump toward rebirth 12 through 20. Use the requirement printed on the pad, not a copied milestone, if the label differs.