Slant hole drilling. The technique of intentional. controlled drilling at an angle from the vertical by deflecting the drill bit. Although wellbores are normally drilled vertically, it is sometimes necessary or advantageous to drill at an angle from the vertical. Controlled directional drilling makes it possible to reach subsurface areas laterally remote from the point where the bit enters the earth. It involves the use of turbodrill, whipstocks, or other deflecting tools.