Make sure you don't confuse this with Yohimbe bark, which can be tough to dose correctly and can often cause nausea or sickness. Yohimbine is also a stimulant, but it's more specifically known as an "alpha-2 adreno receptor antagonist."
It's unique in its ability to essentially "block" alpha 2 receptors within the fat cells, and stubborn body fat areas are linked to sites rich in these alpha 2 receptors.
The alpha 2 rich areas are known to have a poor blood supply, so even when we exercise it's hard to mobilize and move these fatty acids. They also actually appear to inhibit fat loss via signaling of HSL (Hormone Sensitive Lipase), which is a key fat-releasing enzyme in the body. So theoretically, if we can try to switch off these alpha 2 receptors, we may increase blood flow as well as switch on lipolysis (fat burning) in the stubborn areas.
I've used yohimbine on my clients and myself to get us into competition condition multiple times and have always noticed a significant difference in both the speed and the magnitude of fat loss with yohimbine. It's now a staple in my fat loss arsenal and I'm genuinely surprised that more people don't use it, by itself or as part of a formulation.
A popular time to take yohimbine is first thing in the morning on an empty stomach, 20 minutes or so before your morning cardio. (This is because the effects of yohimbine are negated by food intake, especially surges in insulin.) If you can't do your cardio fasted for some reason, make sure you take it around 2-4 hours after a non-carb meal to allow the effects to ramp up.
As for dosing, the most effective dose of yohimbine is around 0.2mg/kg. But start with a lower dose than that, especially if it's part of fat-loss formulation that contains other ingredients. If you're using it by itself, titrate the dose up gradually. Remember that yohimbine is a stimulant and can cause jitteriness.