Marangoni effect in COMSOL

Min

I'm trying to understand the implementation of the following boundary condition at a liquid/air interface using COMSOL: eta*{\partial u/\partial y}=gamma*{\partial T/\partial x}. If lm is the dependent variable in a Weak Form, Boundary applicatoin mode, I think the weak formulation is lm_test*(eta*uy-gamma*Tx). But the expression given in the guide is: lm_test*(eta*uy-gamma*Tx)+u_test*lm. In the guide it says the second term is to make sure this condition replaces the condition from Slip/Symmetry boundary condition in the Incompressible NS application mode. Can anyone kindly explain how this works?