TrickMandelstam[expr, {s, t, u, m1^2 + m2^2 + m3^2 + m4^2}]
simplifies all sums in expr so that one of the Mandelstam
variables s, t or u is
eliminated by the relation s + t + u = m_1^2 +
m_2^2 + m_3^2 + m_4^2 . The trick is that the resulting sum has
the most short number of terms.
ClearAll[s, t, u]
(s + t - u) (2 SMP["m_W"]^2 - t - u)
TrickMandelstam[%, {s, t, u, 2 SMP["m_W"]^2}] // Factor2(s+t-u) \left(2 m_W^2-t-u\right)
-2 s \left(u-m_W^2\right)
M^2 s - s^2 + M^2 t - s t + M^2 u - s u
TrickMandelstam[%, {s, t, u, 2 M^2}]M^2 s+M^2 t+M^2 u-s^2-s t-s u
2 M^2 \left(M^2-s\right)