Ann: Change in substantial holding, page-11

One doesn't ever unload everything at once. It can cause panic and you certainly don't want to break key levels with selling high volume because then the remaining shares lose a lot of value.

if you want an example of this check out RAP in 2015 where the shareprice tanked from its peak. Amateur University staff who did work for RAP nd got paid in shares unloaded heavily across 3 or 4 days (from memory) and sparked a massive sell off. Eventually recovered though once an announcement clarified where the selling came from.

Big holders typically instruct brokers to sell and bots are used to unload certain quantities of shares, say 10,000 per half hour maximum, as an example. Offloading hundreds of millions of dollars worth of shares is very, very challenging to do.. you need a buyer on the other end of all those shares you're offloading.