Pulse Sequences in NMR: Understanding π/2 and π Pulses

[einstein1921]

HI,my dear friends! I have find some materials about π/2 pulse and π pulse ,but I still can't understand the "physics" behind these concept! Anyone can shed light on this problem would be highly appreciated!Thank you!

 

[mfb]

The animation at Wikipedia (+precession) shows how they work. Alternatively, take any introduction to NMR and ask for specific parts that are unclear.