Closure of aortic and pulmonic valves just after systole.
Persistently split S2 (widened S2) is due to pulmonic stenosis, PE, or RBBB (all cause delayed function of right ventricle.)
A young girl with a split S2 and with RBBB on ECG is Pulmonic Stenosis. Not ASD or VSD.
Normally heard as split during inspiration. The split disappears on expiration.