What do you mean "hold on to them"? Teleport needs root privileges to create sessions like `ssh root@host`, like any SSH server would. But DROPS privileges to start the session, i.e. when they no longer needed.

I am assuming you and hackcasual aren't familiar with Golang, since you aren't seeing familiar fork()/setuid()? I'll try to explain:

"dropping privileges" term comes from the old tradition of forking a child process from a privileged parent. Teleport, instead of calling fork() directly as you'd probably expect C code to work, uses Golang's syscall.StartProcess(), which does the entire fork-and-drop privileges logic, and the new privileges (user-specific ones) are passed as shown in the line of code above.

User SSH sessions are sandboxed in unprivileged processes just like you'd expect.