When speaking in terms of herbal "energetics", there are many classifications that can be used. These are helpful in defining herbal actions because they addresss the plant remedy from a holistic point of view, giving a broader "feel" of the herb's medicinal virtues. Sometimes (though not always), these classifications bear a striking connection to physical / biochemical characteristics - for instance, "yin" herbs tend to be more cooling, soothing, demulcent and rich in mucilage; "blood (xue) tonic" herbs may have a good supply of iron, or serve to increase erythropoiesis. Recently, an interesting study out of Hong Kong highlighted a potential biochemical connection to the energetic idea of "yang", always associated with warmth, increased vitality, and energy.
A systematic review of some "yang" botanicals showed that they invariably increase mitochondrial ATP-generating capacity. Deep inside our cells, the mitochondria create a proton gradient between two membranes by moving electrons through the electron-transport-cycle. This gradient is used to drive a transmembrane protein, ATP-synthase, which attaches a phosphate group to spent ADP, renewing its energy-generating ability. The protons can also move back down the gradient through "leak" channels, akin to a short-circuit, instead of through the ATP synthase protein. This has the net effect of generating warmth, and serves as a basic temperature-regulating mechanism for the body. So, it seems, "yang" herbs do in fact increase energy, vitality and warmth. But, as chinese medicine rightly points out, they might be contraindicated in severely depleted conditions (can't stoke the furnace without enough fuel) or in highly overheated, overstimulated conditions (obviously).
File this under the "science finally catches up" category.