def METHOD_NAME(self, toas, param, acc_delay=None):
    """Derivative of delay w.r.t. FDJUMP parameters."""
    assert (
        bool(self.param_regex.match(param))
        and hasattr(self, param)
        and getattr(self, param).quantity is not None
    ), f"{param} is not present in the FDJUMP model."
    y = self.get_freq_y(toas)
    mask = getattr(self, param).select_toa_mask(toas)
    ymask = y[mask]
    fdidx = self.get_fd_index(param)
    delay_derivative = np.zeros_like(y)
    delay_derivative[mask] = ymask**fdidx
    return delay_derivative * u.dimensionless_unscaled