John, it is a load sensor - as the aerodynamic backloads increase and the non-rotating star is pushed down onto the top of the servo jacks, a force that exceeds 330daN will cause a switch to close, illuminating the LIMIT light and sounding the gong.
This also occurs with pressure loss in the RH hyd system which feeds the upper half of the jacks.
Note that it is the jack body that moves up and down whilst the fescalised shaft is bolted to the MRGB