Depending on the server software thats running the ircd, if a channel is +i'd then a user wishing to enter can knock on the channel with /knock #blah This will send an alert to the op's of the channel and then they can /invite JoeUser into the channel, However some bot floods are getting more and more clever and the bots will knock like mad on a i'd channel, So also if an op sets mode +K then knocks are not allowed on a certain channel