A .init section is a piece of code belonging to a shared object that is executed when the shared object is loaded. For example, the .init section is used by the C++ runtime system to call all static initializers in a .so.
The dynamic linker first maps in all the shared objects, putting them on the link map. Then, the dynamic linker traverses the link map and executes the .init section for each shared object. The syncrtld event (see syncrtld) occurs between these two phases.