During the last decade, countercurrent shear has been established as an effective flow control technique for increasing turbulent mixing in a variety of flow configurations and operating regimes. Based on the robust mixing enhancement observed for jets and shear layers, the technique appears to have many potential benefits for enhancement and control for turbulent combustion flows. Countercurrent shear flow control has been applied to a planar asymmetric rearward-facing step dump combustor. A nonreacting flow study on the implementation of suction-based countercurrent shear at the dump plane provided insight into the flow control mechanisms. Control of turbulence velocity and length scales occurs through two mechanisms, the development of a countercurrent shear layer near the dump plane, and enhanced global recirculation caused by the removal of mass at the dump plane. Parametric studies on the geometry of the suction slot indicate that the enhancement of the global recirculation zone is the primary mechanism for increasing global turbulence levels within the combustor. Turbulence energy and length scales both increase in a manner such that the spatially-filtered strain rates as measured with particle image velocimetry remain nominally constant, a desirable characteristic for premixed turbulent combustion. Connections will be made to a recent study on fully-developed turbulent countercurrent shear layers showing additional attractive features of countercurrent shear including enhanced turbulent energy production, entrainment, and three dimensionality. Preliminary reacting flow results for the dump combustor operating while burning premixed/prevaporized JP-10 illustrate qualitative changes in the turbulent combustion process within the combustor. The companion paper will describe the quantitative effects of countercurrent shear on the global heat release rates within the combustor..