L-theanine promotes focus and a calming effect because it is a natural building block for the GABA neurotransmitter. For info about the chemical structure of l-theanine read this Wiki page.
GABA is an important regulating neurotransmitter.
L-theanine consumption is effective for improving focus and memory.
Research shows that men and women who suffered minor brain injuries develop improved memory during a cognitive exercise after they consumed l-theanine.
L-theanine makes you become more alert and maintain focus for a longer period (Study).
Combining l-theanine and caffeine: a positive synergistic effect.
The positive caffeine-effects are noticeable for a longer period while the negative effects of caffeine become less noticeable.
L-theanine is a very effective natural nootropic agent.