Specifically, the more recently the channel was expanded in one direction, the longer the lookback period grows in that direction. Conversely, if the channel has not been expanded in a given direction, the lookback period will contract so as to allow for a tighter channel.
For example, let the initial lookback period be 20 bars and let the factor argument be 0.1 (or 2 bars to start, as 20*0.1 = 2). Now say the current bar sets a new 20-period high. Then the lookback period for the upper bound is expanded by 2 bars to 22, and the lookback period for the lower bound is contracted by 2 bars to 18, thereby making it simultaneously harder to set new highs and easier to set new lows (and vice versa for hitting new lows). If neither a new high nor a new low is formed, both periods contract by the given factor.