function onStartBgPos(prop) {
    if (this.valuesEnd[prop] && !KEC[prop]) {
      KEC[prop] = function (elem, a, b, v) {
        /* eslint-disable -- no-bitwise & no-param-reassign impossible to satisfy */
        elem.style[prop] = ((numbers(a[0], b[0], v) * 100 >> 0) / 100) + "%  " + (((numbers(a[1], b[1], v) * 100 >> 0) / 100)) + "%";
        /* eslint-enable -- no-bitwise & no-param-reassign impossible to satisfy */
      };
    }
  }
Sets the property update function.
@param {string} prop the property name