There are two herbals in Thailand that use the name krachai (กระชาย). They are krachai dam (กระชาย ดำ, black krachai), and ton krachai (ต้น กระชาย, krachai trunk). Contrary to what the similar Thai common name suggests, the two plants are botanically not related to each other.
But both are medicinal plants, and Thai ethymology sorts medicinal herbals by their pharmacological properties.
Know some Thai, here we go!
The Thai word chai (ชาย) means male. The word for female is ying (หญิง). The prefix kra (กระ) cannot be translated easily. It's a common noun prefix. The prefix ton means trunk. Trees and lumber logs are counted as ton (a grammatical plural in Thai, no have). No connection to the word for 1000 kg.
So, krachai means "male thing". And indeed, no other name could be more befitting. Because the effect of these two herbs are on that certain male thing.
By the criteria of Western pharmacology, both krachai dam and ton krachai are phosphodiesterase inhibitors. You know what that means. Krachai dam and ton krachai are in the same class as sildenafil, the active ingredient in Pfizer's Viagra.
Here sound scientific evidence:
Potent Phosphodiesterase Inhibition and Nitric Oxide Release Stimulation of Anti-Impotence Thai Medicinal Plants
Key sentence: Boesenbergia rotunda (L.) Mansf. extract [EDP1-001(1)] gave the highest PDE inhibition activity of 4.36-fold sildenafil, a standard anti-impotence drug.
Kaempferia parviflora, a plant used in traditional medicine to enhance sexual performance contains large amounts of low affinity PDE5 inhibitors
Kaempferia parviflora rhizome extract and its 7-methoxyflavone constituents had moderate inhibitory activity against PDE5. This finding provides an explanation for enhancing sexual performance in the traditional use of Kaempferia parviflora.