def _drop_preTTL(NPM_df):

    """
    Helper function that finds the first TTL trigger and drops the data before it.

    Parameters
    ----------
    NPM_df : dataframe
        dataframe containing the either NPM data with Flags.

    Returns
    -------
    NPM_df_short : dataframe
        dataframe without the pre-TTL data and reset indexes.
    """

    TTL_idxs = NPM_df.index[NPM_df['Flags'] > 20]
    First_TTL = TTL_idxs[0]

    NPM_df_short = NPM_df.iloc[First_TTL-1:].reset_index()
    return NPM_df_short