Edit: A slight correction - of course if there is 10 BTC to be sold for $800, and someone buys only 0.5 ... the price will remain $800 until all of those are sold and we move up the order book.
Okay that's what I was wondering. So a whole offer needs to be accepted and a complete sale needs to be made before that incremental price increase registers.
Depends. Let's say someone is selling BTC for $800 (ask). Someone else is willing to buy BTC for $790 (bid).
If someone holding BTC is willing to sell to that $790, the price drops. But if someone buys even 0.01 BTC for $800 after that, the price immediately goes up to reflect the most recent trade.
He only bought BTC for a fraction of that $800, but it still took a piece out of that sellers pie.
Now, to go past $800, someone would need to buy the entire pie.