Adding Phase Shifting Transformer to Y matrix

When adding a shift phase transformer,

The code shown below is used to modify the admittance matrix using MATLAB Code.

t=1.2;

n=1 ; %from

m=2 ; %to

yl=-y(n,m);

y(n,m)=y(n,m)+yl*(1-(t^-1));

y(m,n)=y(m,n)+yl*(1-(t^-1));

Can someone express how to get the equations above (lines 4 to 6) ?