The trigger points vary on the volume and price of the stock in question.

FORBES: Some Agreement on Speed Bumps for Market Panics

Can use the volume up button as a shutter trigger.

FORBES: Apple: Live From The WWDC; OSX Lion, iOS 5; iCloud

Some 70% of all equity volume is driven by computer models that suddenly trigger a surge in buy or sell orders that are causing the fragmentation, volatility and use of hidden trading venues.

FORBES: Watch Out For The Quants