Omega,
I have arrived on the first bus as in this part of the world (Sussex) the trains seldom run. You are correct it is the anti swing doing 'what it says on the tin'. Due to the low drop height of reefed mains MSP/HSP without such a device
the load would not have time to stabilise before landing on the DZ. I have seen what happens to the load when the anti swing does not work as advertised. Not pretty !