This is the Russell 2000 which on Wednesday gained over 4% in 20 minutes.
I haven’t really heard an explanation for this other than it might have been a short squeeze. Although the spectre of HFT has raised its head.
What this also demonstrates is the asymmetry of common sense possessed by regulators. It is an affront against nature for an instrument to go down by 4% in 20 minutes but should an instrument do the reverse this is considered normal and not worth commenting on.