Residual Connections Keep Each Block's Jacobian Near the Identity

… more less For \( x_{\ell+1} = x_\ell + f(x_\ell) \) the Jacobian is \( I + J_f \), so \( \prod_\ell (I + J_{f_\ell}) \) keeps a direct identity path and the gradient reaches early layers undiminished.