A Buy Stop order in forex is used to lock in an exact price of an instrument and a Buy Stop order must be placed above the current price.
So, if you want to buy when price starts rising, you set a Buy Stop at the price you want, so that when price reaches the order, it is triggered at the exact price. My emphasis is on "exact price".
So let us assume you want to buy XAU/USD when the price rises to 2860.0, all you have to do is set the buy stop order at that price.
If due to volatility, the price suddenly jumps to 2862.0 without passing through 2860.0, then your buy stop order will not be executed because it works with a specific price. This is the main difference between a buy stop order and a buy limit order.
A buy limit order would have been filled at either the exact price or a better price.