With a hydrostatic transmission (HST), as your traction load increases (i.e. pulling a heavy load, operating with a loader in a dirt pile, pulling a trailer up a hill or operating in soft soil conditions), remember to maintain or decrease the HST pedal pressure as the load increases.  Do not "give it more gas".  

The hydrostatic transmission operates on the principal of hydraulics using a pump and a motor. Under heavy load conditions, pushing down on the pedal causes the pump-to-pump more oil, thereby increasing oil flow, which the motor may not be able to accept.  In this case, the HST will "stall" as the HST relief valve temporarily opens, or the engine may stall if the hi/lo range lever is in high range. 

The increased oil flow from the HST pump gets dumped off as the relief valve opens.  This causes the HST motor to slow down or even stop rotating.   Maintaining the HST system's oil pressure within its operating range allows the hydrostat's motor to do the work more efficiently.  When operating in heavy load conditions, it is recommended to gear down to "lo" range, which also allows the HST to be more efficient.