That's how you do it though. You place sell orders in...

That's how you do it though. You place sell orders in pre-auction and if nobody puts bids up to match the sell at what you are after, you pull the sells and drip feed them out during the day or in match at end of the day. Simply leaving massive sells there is counterproductive.

Brokers do this for clients looking to buy or sell large amounts of shares so they can avoid moving the market significantly either way.