Called MBNA just now.
About 你马上得到$50申请现金Rebate, seems they don't have this anymore.
They told me when you have earned enough points to get 50 dollars, they will send you a check of 50 dollars. For example, if you spend 600 on gas and grocery in the first month, 600 * 5% = 30 dollars, next month, when you spend 400 on gas and grocery, 400 * 5% = 20 dollars, 30 + 20 = 50, they will send you a check of 50 dollars.