the key point is letting the engine drag your vehicle into the certain speed.
let say you are driving in 4th gear with 100km/h, when you stopping putting your foot on the gas, you are letting the engine drags your vehicle to the speed that engine idling in 4th gear.
but if you are driving in 6th gear with 100km/h, simply just shift down to 5th gear letting the engine to drag your vehicle down by shifted into 5th gear.