I will discuss a new observable that probes the couplings of the light and top quarks to new physics. The observable is the threshold lepton asymmetry, that is the forward-backward asymmetry of the charged lepton in ttbar events near the production threshold. At threshold, angular momentum conservation implies that the top spins are uniquely fixed by the chirality of the initial quarks. Thus, measuring the final state top spins determines the chirality of the quarks which produced them. Information about the top spins can be extracted by measuring the threshold lepton asymmetry. This observable vanishes in tree-level QCD but is non-zero if new physics modifies the relative contribution of right-handed and left-handed quarks to top pair production. Models addressing the anomalous ttbar forward-backward asymmetry at the Tevatron always involve chiral couplings to light quarks. Models predicting identical ttbar forward-backward ̄asymmetries can be distinguished by their threshold lepton asymmetries which range between plus and minus 25% in realistic situations.