  The mechanism behind the nematicity of FeSe is not known. Through
elastoresitivity measurements it has been shown to be an electronic
instability. However, so far measurements have extended only to small strains,
where the response is linear. Here, we apply large elastic strains to FeSe, and
perform two types of measurements. (1) Using applied strain to control
twinning, the nematic resistive anisotropy at temperatures below the nematic
transition temperature Ts is determined. (2) Resistive anisotropy is measured
as nematicity is induced through applied strain at fixed temperature above Ts.
In both cases, as nematicity strengthens the resistive anisotropy peaks about
about 7%, then decreases. Below ~40 K, the nematic resistive anisotropy changes
sign. We discuss possible implications of this behaviour for theories of
nematicity. We report in addition: (1) Under experimentally accessible
conditions with bulk crystals, stress, rather than strain, is the conjugate
field to the nematicity of FeSe. (2) At low temperatures the twin boundary
resistance is ~10% of the sample resistance, and must be properly subtracted to
extract intrinsic resistivities. (3) Biaxial inplane compression increases both
in-plane resistivity and the superconducting critical temperature Tc,
consistent with a strong role of the yz orbital in the electronic correlations.
