Complex sentence: If an airfoil's vortex produces downwash, somewhere else the airframe must also produce a similar amount of upwash, in "stable flight" this occurs approximately within the airframe surface envelope; it can also occur outside of the airframe envelope under special circumstances.
Split into simpler sentences: If an airfoil's vortex produces downwash, somewhere else the airframe must also produce a similar amount of upwash. In "stable flight" this occurs approximately within the airframe surface envelope; it can also occur outside of the airframe envelope under special circumstances.