Final answer:
The most gentle method of locking hair is thermal locking, which uses heat to temporarily change the hair's structure without the use of chemicals.
Step-by-step explanation:
The method of locking that is most gentle on the hair is thermal locking. In thermal locking, heat is used to temporarily change the structure of the hair without the use of chemicals. For example, using hot rollers or a curling iron to create curls is a form of thermal locking. This method is less damaging to the hair compared to chemical methods such as perming or straightening, as it does not involve exposing the hair to harsh chemicals.