public/javascript/libs/wysihtml5-0.3.0.js
function(avoidHiddenFields) {
      var field,
          fieldName,
          newValue,
          focusedElement = document.querySelector(":focus"),
          fields         = this.container.querySelectorAll(SELECTOR_FIELDS),
          length         = fields.length,
          i              = 0;
      for (; i<length; i++) {
        field = fields[i];
        
        // Never change elements where the user is currently typing in
        if (field === focusedElement) {
          continue;
        }
        
        // Don't update hidden fields
        // See https://github.com/xing/wysihtml5/pull/14
        if (avoidHiddenFields && field.type === "hidden") {
          continue;
        }
        
        fieldName = field.getAttribute(ATTRIBUTE_FIELDS);
        newValue  = this.elementToChange ? (this.elementToChange[fieldName] || "") : field.defaultValue;
        field.value = newValue;
      }
    }
function(avoidHiddenFields) {
      var field,
          fieldName,
          newValue,
          focusedElement = document.querySelector(":focus"),
          fields         = this.container.querySelectorAll(SELECTOR_FIELDS),
          length         = fields.length,
          i              = 0;
      for (; i<length; i++) {
        field = fields[i];
        
        // Never change elements where the user is currently typing in
        if (field === focusedElement) {
          continue;
        }
        
        // Don't update hidden fields
        // See https://github.com/xing/wysihtml5/pull/14
        if (avoidHiddenFields && field.type === "hidden") {
          continue;
        }
        
        fieldName = field.getAttribute(ATTRIBUTE_FIELDS);
        newValue  = this.elementToChange ? (this.elementToChange[fieldName] || "") : field.defaultValue;
        field.value = newValue;
      }
    }
Takes the attributes of the "elementToChange"
and inserts them in their corresponding dialog input fields

Assume the "elementToChange" looks like this:
<a href="http://www.google.com" target="_blank">foo</a>

and we have the following dialog:
<input type="text" data-wysihtml5-dialog-field="href" value="">
<input type="text" data-wysihtml5-dialog-field="target" value="">

after calling _interpolate() the dialog will look like this
<input type="text" data-wysihtml5-dialog-field="href" value="http://www.google.com">
<input type="text" data-wysihtml5-dialog-field="target" value="_blank">

Basically it adopted the attribute values into the corresponding input fields