Why do riders raise their hands in case of an accident?

I always see riders raise their left hand when they see a crash in front of them. What is that signaling? And is there other gestures they could do during a race to signal something? To their team or other riders?
Submitted October 30, 2018 at 10:09PM by FormulaHaydraa https://www.reddit.com/r/motogp/comments/9ssn26/why_do_riders_raise_their_hands_in_case_of_an/?utm_source=ifttt