Higher pressure is utilized to deform the fitting and compress the sealant in opposition to the internal tubing, making a leakproof seal. Underground piping methods for landscaping drainage or even the disposal of stormwater or groundwater also use minimal-strain gravity movement, so fittings for these systems resemble larger-scale DWV fittings. http://traviseoxek.collectblogs.com/18014478/the-ultimate-guide-to-pipe-fitting