If you see attacks like this
Then send reinforcements to arrive EXACTLY at 04:52:18. They will arrive after the attacks at 04:52:18 but before the attacks at 04:52:19 (first sent attack or reinforcement is the first to arrive...you send reinforcements after attack is sent, so reinforcements arrive after the attack). This is difficult to hit exact timing, but if you have multiple players trying to do it, one will make it at the right time (cancel it if you accidentally send too early).
If the attacks are spread further apart, it's much easier:
Then you see that you have 8 seconds between 04:52:18 and 04:52:26 to have reinforcements arrive.
This is the whole purpose in trying to send multiple attacks in the same second, as described in this thread
. If your attacks are spread apart by several seconds, that makes it very very easy for the defender to insert troops and kill your later attacks.