Basically, it is an expected correction on a brutal fall in prices. In the market jargon, it is a trap for the bull traders.
Bulls, Stay Alert!
(a) The future will change the value of our shares.
(b) There will be no room for all!
(c) No! They will loop us in the charts!
Your observation has been corrected, the graph is very similar to the Dow Jones Crash in 1928 (-45% in 1928 - bounce up - and a free fall at -90% ended in 1934) also happened to the NASDAQ in 1998 (-38% down, bounce up - and finally -78% crash put down in 2004) and in the Bicoin/USD graph in 2017 (-35% down - bounce up and -70% teminated in 2018) and in many others charts.
In these weeks the first flash crash in the Sp500 chart stopped at -35% - bounce up - and ...?
We'll see in the next weeks if the cat was really dead and where will it rest in the next months or years