Reincarnation is a very wide topic.
There are many different religions that believe in reincarnation, and in each it is explained differently.
If you are asking my personal belief then i would say that reincarnation is not instantaneous. For a person ( a spirit ) to reincarnate it has to create connections to a physical body, and this is process that starts roughly at conception and it is roughly finished by the time of birth.
Different people have different beliefs on this regard though.
There are many different religions that believe in reincarnation, and in each it is explained differently.
If you are asking my personal belief then i would say that reincarnation is not instantaneous. For a person ( a spirit ) to reincarnate it has to create connections to a physical body, and this is process that starts roughly at conception and it is roughly finished by the time of birth.
Different people have different beliefs on this regard though.