Javadocs for Explicit would help :). The way I read it coerce is a Boolean initialized to null. If someone uses calls Builder.coerce(boolean) with any value they are making an explicit choice of setting and coerce is then non-null. Then, in the Builder.coerce(BuilderContext) method I follow the cues from the existing IgnoreMalformed settings - either take an explicit choice set in the Builder if coerce!=null or fall back to an index-wide default which is a non-explicit choice (hence the false param).
The explicit choice is then used to decide what takes preference when merging mappings - you prefer the value setting defined in the mapping the made an explicit choice over coerce settings to one that just ran with the implicit default.