I assume he means he uses git worktree to create a directory structure like so:
src/my_project_master/
src/my_project_develop/
src/my_project_new_feature/
Each with it's own IntelliJ project. If he needs to switch branches it's relatively fast to open a different existing project rather than git checkout branch, which can cause IntelliJ to become unresponsive as it reindexes everything.
Not tried this, but I experience this quite often and will have to give this a shot.