Hi there, fellow nerd. I, too, prefer peer-reviewed journal articles to vague claims. Here’s just a sampling of links explaining the marriage of l-theanine and caffeine:
“The L-theanine and caffeine combination improved both speed and accuracy of performance of the attention-switching task at 60 min, and reduced susceptibility to distracting information in the memory task at both 60 min and 90 min. These results replicate previous evidence which suggests that L-theanine and caffeine in combination are beneficial for improving performance on cognitively demanding tasks.”
http://www.maneyonline.com/doi/abs/10.1179/147683008X301513
“The combination of moderate levels of L-theanine and caffeine significantly improved accuracy during task switching and self-reported alertness (both P < 0.01) and reduced self-reported tiredness (P < 0.05)”
http://www.maneyonline.com/doi/abs/10.1179/147683010X12611460764840
“The results support previous evidence that l-theanine and caffeine in combination can improve attention.”
http://www.sciencedirect.com/science/article/pii/S0195666310000267
“L-Theanine is an amino acid found in green tea leaf and in its infusion, and is known to control excitement caused by caffeine.”
CalmBrew 