Dividing up a time series into fixed time periods (say, the period of a pulsar) and then aligning them all and adding all the signals so that the nth sample in each period is added together. This will increase the signal-to-noise of a signal which reoccurs at the time period chosen (or, to a lesser extent, at a multiple of that time period. With many folds, the pulse profile is stable despite highly variable appearance in individual pulse periods).