Quote from RedTankEra:
You keep the old channel to determine if the breakout is for real or a head fake as backtests usually develop as important tellling points in the markets. These backtests can occur on the channel formation itself, it's horizontal breakout line point or old formations that you must monitor during the breakout phase.
You can not only create channels/lines off highs and lows, you can also create them off shoulders.
Notice what price did after it broke the downtrend channel, it did not resist at the second shoulder line test, on the contrary, it broke it, and then found support by turning diagonal resistance into diagonal support which is precisely why we meant to breakout and not create a head fake aka breakout failure.
I find horizontal lines less reliable in markets because the market moves in waves, therefore I prefer my support and resistance to be of the diagonal wavery nature.