Hydro distillation, or steam distillation, is often used to extract volatile compounds. Hydro distillation uses steam that permeates the plant containing the desired terpenes. After the steam passes through the plant bringing along the terpenes, the steam is then condensed, creating a water and terpene distillate mixture. The the water is then drained, only leaving the terpene distillate.
Vacuum distillation is an offshoot of Hydro distillation, but offers a much more sensitive extraction method. Vacuum distillation lowers the boiling point of the water used in steam distillation by sucking out part of the atmosphere in the distillation set up. This maintains lower pressure and provides a more pure, less warped terpene.