They sometimes switch from $y^2 = x^3 + 7$ to $y^2 = x^3 + 7 t^6$ in order to use Jacobian coordinates. I haven't looked at why this is useful or whether a similar optimization applies to Pallas and Vesta. (The same structure of isomorphisms between curves is present; the question is whether it is useful for us.)
They sometimes switch from $y^2 = x^3 + 7$ to $y^2 = x^3 + 7 t^6$ in order to use Jacobian coordinates. I haven't looked at why this is useful or whether a similar optimization applies to Pallas and Vesta. (The same structure of isomorphisms between curves is present; the question is whether it is useful for us.)
https://github.com/bitcoin-core/secp256k1/blob/60556c9f49a9384efd7f16b734820ae19108f053/src/ecmult_impl.h#L83-L93