[webkit-help] help for enum SynchronousMode { type means?

[李兵]

  In the class of HTMLDocumentParser have the enum SynchronousMode  type:

enum SynchronousMode {
        AllowYield,
        ForceSynchronous,
    };
    void pumpTokenizer(SynchronousMode);
    void pumpTokenizerIfPossible(SynchronousMode);

but I am don't kown the means of AllowYield and ForceSynchronous?