4-4 point low approach low extension, contact
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The solid connection of is a way to cool down the local situation rapidly. According to current (2021) understanding of AI evaluation, White settles (too) easily and has many good options later, with a powerful shape at a and territory at b.
Still this is playable for Black, as he gets sente, and can be suitable depending on the context.
The more active answer and current standard is to atari at . After Black plays a or b to keep White pressed down to the top side, or c to take positions on both sides at the expense of the corner.
When Black holds back to this solid connection, he can revert to the previous variation, but also decide to take the corner with . Next White usually continues at and Black can exploit the aji at a, which leads to a longer fighting variation.
Black can push through at if there's support on the upper side for the fighting that will occur due to White's cutting stones.
White can also choose to capture the corner directly at . This gives Black a thicker influence but after -, White can take sente.
Playing the bulge for ko is locally bad for White. Black will take and it will require a good ko threat to get advantage out of it. As this is an early corner pattern, such big ko threats are rather rare.