burnout解释的两处不太明白

问题如下：

Explain what is meant by burnout.

解释：

Burnout occurs when interest rates decline over a period of time. Those mortgage holders who are most likely to prepay (e.g. because their mortgage balances are high, they have good credit ratings, or are financially sophisticated) do so during the first part of the period and leave the pool. The prepayment rate for the mortgage holders who are left is then lower.

解释一下什么叫做burnout。

当利率下降时，会出现burnout。此时mortgage的第一部分的持有人会提前还款，离开资产池。那么提前还款率之后就会下降。