These Fibonacci levels are redrawn every time a -23.6% line or 61.8% line is broken. On this 15 minute chart the fibs are based on 120 minute bars to reduce the noise of the 15 minute time frame - blue arrows show when fibs were redrawn due to a close beyond the green or red lines of the prior set of fibs.
Based on past observations, price will likely cycle between 50% & 0% until something changes in fundamentals that causes price to break the green or red lines. Sometimes - when nothing much is going on - the 38.2% line will provide support/resistance, so it is turned on for this chart. It never seems to carry as much weight as the 50% line in terms of support/resistance.
In accordance with my personal trading plan, the 50% retrace (yellow line) will be a potential short entry, -23.6% (green line) a potential long entry.