B1 is the best way to proceed, because of the threat it makes to atari next at 2.
W2, B3 and W4 are then forced, after which Black has 5 liberties and is technically “contact stable”.
But here that’s not adequate, because if Black tenukis, the turn of W5 will be too good for White to allow.
So B5 is necessary, yielding 6 liberties. And because a following B6 would be bad for White, W6 is equally necessary. So Black retains Sente, allowing him to safely tenuki if he desires.