Tethers are thin tubes of lipids (~20-200 nm in diameter) that form when membranes are subjected to a point force. Tether dynamics are important to a myriad of biological processes including white blood cell adhesion and transport of intracellular material between neighboring cells. To understand the dynamics of tether formation more fully, we investigated the dependence of the force needed to create a tether on the rate of force change (loading rate). To conduct these experiments, a microfabricated magnetic force transducer was used to generate well-controlled and localized magnetic force profiles. Tethers were formed off the surface of microaspirated giant unilamellar vesicles (GUVs) attached to magnetic beads. We discovered a strong correlation between the threshold force of tether formation and the applied force ramp, with the force changing from <10 pN at low loading rates to~50 pN at high loading rates. At slow loading rates, the threshold force changes weakly with ln (loading rate), while at high loading rates a steeper dependence is observed. The experimental data can be fit to a energetic model based on Kramer's theory, similar to models used to describe membrane rupture. The model predits that tether formation involves passage over two energy barriers and enbales characterization of the characteristic forces and timescales associated with these barriers. This new tool for dynamic studies of membrane mechanics may further be extended to study how tethers form off of flowing cells or how phase regimes, induced by the presence of cholesterol, influence membrane dynamics.