The idea is to block the two cysteines so they can't react in the future. We need the reaction to remove the -SH groups of the two cysteines, or modify them.
Also important, the reaction should not make cross-link between the two cysteines.
To explain briefly my motivation: I am a PhD student and I do optical tweezer experiments. We can hold and manipulate two micrometer beads in two optical traps. We have a process to tether one protein using two DNA linkers to the two beads. I need to block two of the three cysteines of my trimer to properly couple the protein to both beads.