要注意，应付账款的变动（而不是应付账款本身）涉及到现金的流进或者流出了。比如说，应付账款减少，说明拿现金归还了部分应付账款，所以要减去应付账款的减少。

是呀，应付账款减少已经理解了，但是应付账款增加呢

short answer is that it is the rule, better to memorize.
If you want to know the intuition, let's consider the accounting equation:
      A = L + SE
=> change in A = change in L + change in SE
=> change in cash + change in non-cash assets = change in L + change in SE
=> change in cash = change in SE + change in L - change in non-cash assets

net income results in a change in SE, that is why we need to start from the net income.
change in L includes the change in accounts payable
change in non-cash assets includes change in accounts receivable.

Here the change can be either an increase or decrease. Regarding your question, if you have a increase in accounts payable, you can consider it as you purchased more inventory on accounts than the amount you pay to your supplier. You may somehow sell the extra amount of inventory for cash, which results in the increase in cash.

Hope this helps.