If I understood correctly, if a pilot with bulwark ends the turn out in the open (no terrain cover) activating bulwark (so moving+bracing, not moving or moving and/or shooting+Vigilance), the 'Mech will receive a 20% damage reduction (instead of the 50% from the actual version). 40% damage reduction if the 'Mech is within a terrain that usually gives 25%, e.g. forest, I think that this is kind of passive. And the 60% if bracing within cover terrain (forest, windwhirls..). I'm sure that either in-game text and/or release notes will completely clarify it.