Just think of it this way, when you depart and climb your tank fuel is aerated (contains air), during a rapid climb the air within the fuel will expand and if the fuel is not delivered under pressure then the expanding air can disrupt flow and cause engine flameouts. Below FL150 the expanding air is not sufficient to cause issues. If you have been in a stabilised cruise for more than 30 minutes then the fuel has had a chance to settle and is considered de-aerated. If you are already above FL300 but not for 30 minutes then back to FL300 gives best protection against flameout. So if your two wing tank pumps fail and you are above FL150 then the concern is that the engine will flameout so the ECAM calls for Xfeed open and fuel be delivered to both engines under pressure from the functioning pumps. Once you have established if you have been in cruise above FL300 for more than 30 minutes the fuel,is de-aerated as long as you don’t continue to climb, and you can rely on gravity low pressure feed so xfeed closed. If you are above FL300 but less than 30 minutes then use xfeed open and hi pressure feed until back at FL300, once back at FL300 then Airbus considers the fuel de-aerated so you can close the xfeed and rely on low pressure gravity feed. If you never got to FL300 then back to FL150 for you. The whole point of the procedure is to reduce the risk of flame out when using aerated low pressure fuel, to eliminate that risk you use the high pressure fuel from the good tank using the xfeed. Once you establish the fuel in the low pressure tank is safe to use then you use it by closing the xfeed.