The delayed joins feature allows channel operators to essentially squelch join/part floods.
How does it work?
When a user joins a channel with delayed joins enabled, they're considered to be "observing" and the server doesn't send a join notification to other users in the channel, not even channel operators. If that user sends a message to the channel, changes the channel's topic or a mode change occurs on them (e.g., they're voiced or opped), then they're considered "revealed" and a join notification is sent to your client.
How do I enable delayed joins in my channel?
Any channel operator can enable delayed joins by setting channel mode +D.
/mode #darenet +D
Likewise, unsetting channel mode +D will disable delayed joins.
Note: Disabling the delayed joins feature, while there are still delayed-join (hidden) users in the channel, will cause the server to automatically set channel mode +d in its place; we'll cover that below.
Why is +d set on my channel, and why can't I remove it?
As mentioned above, the channel mode +d is set automatically whenever the delayed joins feature is disabled and there are still delayed join users present in the channel. When all of these users have been revealed or part the channel, the server will automatically remove +d. You cannot remove or set +d on your own, but you can voice those users who were hidden by delayed joins to reveal them.
How do I view delayed-joined (hidden) users in a channel?
You can view users hidden by the delayed joins feature by using the /NAMES command with the -D argument.
/names -D #darenet