A note about this, from /fit/:
You'll want to make sure to not spike your blood insulin levels for three hours after taking the Yohimbine. This means no carbs and, preferably, no food at all, after which a high-protein meal is best.
The Yohimbine HCL opens up a metabolic pathway to your abdominal fat tissue, and increases bloodflow to the abdominal area (broscience explanation, but that's the gist of it). If fasted in this state, any caloric deficiency will preferentially burn ab fat. If you really want to get the most out of it, some light cardio 20 minutes after supplementation will maximize this effect. The converse, however, is also true - an insulin spike will send fat stores straight to your tummy.
I know that's not the point of that part of The Stack, but it can be a synergistic effect if done right.
Alpha Lipoic Acid, incidentally, has a similar but somewhat opposed effect - if taken shortly before a meal, it will send more of the incoming calories (assuming appropriate macronutrients) to rebuild muscle tissue rather than fat storage.