  We report the separate response of the critical temperature of the nematic
phase transition $T_{\rm S}$ to symmetric and antisymmetric strains for the
prototypical underdoped iron pnictide Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$.
This decomposition is achieved by comparing the response of $T_{\rm S}$ to
uniaxial stress and hydrostatic pressure. In addition to quantifying the two
distinct linear responses to symmetric strains, we find a quadratic variation
of $T_{\rm S}$ as a response to antisymmetric strains $\epsilon_{\rm
B_{1g}}$=$\frac{1}{2}$($\epsilon_{\rm xx}$-$\epsilon_{\rm yy}$), exceeding the
non linear response to symmetric strains by at least two orders of magnitude.
These observations establish orthogonal antisymmetric strain as a powerful
tuning parameter for nematic order.
