How does a tipper truck work?

At the front of the dump body, there is a large cylinder in several sections which collapse into the largest section while the bed is down. When the driver is ready to raise the bed, they will normally unlatch the tailgate first (this may be done either manually, or from controls in the cab). Then they engage the PTO (Power Take Off), which actuates a hydraulic pump. This pump pushes hydraulic fluid into that cylinder. As this happens, the individual sections are forced up by the pressure of the hydraulic fluid. Once a section reaches its maximum extension, the next section raises, and this continues until all sections are completely extended. The commodity dumps out of the bed, and the driver usually has to pull forward slightly to ensure it all comes out. Bringing the bed back down once it is empty is normally a simple matter of disengaging the PTO, and then disengaging the hydraulic pump. This releases the pressure which raised the bed. The weight if the dump body is sufficient to push the hydraulic fluid back into the holding tank, and lower itself. some systems do require the PTO to remain engaged, and these will have a system which requires the hydraulic pump to be reversed in order to bring the bed down. This is, however, rather uncommon in regular dump/tipper truck, and is much more common on roll-off container trucks.