what is going when you can buy rise or fall on BTC
You might need to clarify what you mean here. Are you asking "what is a limit order"?
where i can do this? You can set a time limit as well.
If you're asking about limit or stop orders, most exchanges let you do this. You can also specify the price you're willing to pay, list the order and then wait for the market to move to your ask/bid price.