From the beginning of the series, even during his last moments, Itachi poked Sasuke's forehead as a way of showing affection and apologizing.
But is that the only reason? Did he perhaps try to transfer some knowledge or memories to Sasuke? Or is it simply unknown?
It has nothing to do with transferring memory or knowledge.
It's a gesture of love from Big Brother (Itachi) to his loving Younger Brother (Sasuke). That gesture show how much Itachi love Sasuke than anything.
Itachi had always known the “fate” of Uchiha brothers. One would always be destined to take the other’s sharingan. The slow approach of Itachi’s fingers are meant to tease at the idea of what he, the stronger brother, is “supposed to do”, which is to take Sasuke’s eye (sharingan) but he just pokes him in the forehead like an older brother will mess with a younger brother’s hair. It’s a sign that he loves him too much to follow the Uchiha fate.
Itachi always loved Sasuke and it seemed more of a way of affection. I don't believe he can transfer power in that sense, but we do later learn that Chakra can be temporarily transferred via a fist bump so its possible.
I believe it to be a sign of affection and he did it before he died to let Sasuke know that he still loved him even though he was unable to speak at the time as he was extremely sick with what appears to be hemoptysis.
As we know it on the episode that Itachi poke's Sasuke, 'cause Itachi loved Sasuke so much, he's not seen Sasuke from a long time & Itachi's love is much more than anything to Sasuke.
If you want to know more about the affection between Itachi to Sasuke, you can watch the filler arcs about them on episode 451.
This had probably meant for showing love and affection = Itachi to Sasuke. This could not have been some type of jutsu or funny way of saying "not right now". For Itachi maybe, but it states in season 3 that since Sasuke had tapped Sakura, I'm sure he didn't mean to give her any chakra.
Stating above, this could not have been a jutsu. Personally, I think it's just a nice way of telling somebody how much you care for them without doing their bidding because Sasuke had asked Itachi to help him and he wanted to say no in a caring way, just as Sakura told Sasuke to stay, and as Sakura does to their child Sarada to show her caring as shown on these images on PhotoBucket and Tumblr.
Its a friendly gesture of Itachi poking Sasukes forehead, With that itachi wants to let Sasuke know he still loves him after all. But theres a chance that Itachi gave him something Like the last step of the Sharingan. Just keep watching Naruto and you will find out.
When Itachi poked Sasuke's forehead, you can clearly see blood running into Sasuke's eye, the same eye that casts the (black fire jitsu, extremely hard name). The blood was probably from Itachi's eyes, therefore resulting in a possible transfer of visual-based jutsu.
But is that the only reason? Did he perhaps try to transfer some knowledge or memories to Sasuke? Or is it simply unknown?
He did give Sasuke the mangekyou sharingan moments before he died with that symbolic hand gesture.
