why would you lock magic burst mode, what is a mode?
To add on to gab's otherwise excellent answer:
Modes are logic in your lua that change the behavior of the code. While in one mode certain gear might be equipped for a spell but in a different mode, other gear would be chosen instead.
For example: if you're magic bursting a spell you want to prioritize gear with Magic Burst Damage, but if you're free nuking (not Magic bursting) then MBD doesn't do anything so locus ring is terrible.
You might have engaged modes like high DT vs low DT, depending on the content you're doing. High PDL vs no PDL, etc.
He explained why you might want to toggle MB mode, because you're bursting and may want to pre-cast your Magic burst spell, to get more in to the window.
You might also need/want to toggle it in a different lua if, for example, you had less automation. Many luas REQUIRE the user to handle all toggles between modes.