Dust devils, small-scale convective vortices found on Earth and on Mars, can transfer substantial quantities of dust from the ground into the atmosphere. It has been proposed that the low-pressure region found at the center of dust devil vortices provides a lift (the 'ΔP' effect) that 'sucks up' material from the surface. Two simple models are compared to investigate the physics behind the ΔP effect and the relevance of competing processes. The first considers an impermeable bed of particles where lifting is by vertical pressure gradients, the second considers a permeable bed where lifting is by drag forces on the particles as gas is sucked from the bed. Pressure gradient lifting appears to be far more efficient than drag force lifting. We describe conditions that favor lifting by the ΔP effect and make qualitative predictions that might be tested in the laboratory, the field, or through observations from Mars Landers.