http://giggles.vernccvbvyi5qhfzyqengccj7lkove6bjot2xhh5kajhwvidqafczrad.onion/study/learn-from-phoebos.html
To understand what is happening, I choosed to look again on man page of dash, and I found out that exec is a builtin, and its documented under the Builtins section, I will reproduce the interesting part here: Builtins
This section lists the builtin commands which are builtin because they need to perform some operation that can't be performed by a separate process.