I got several blisters from pushing the thumbstud.
But pretty soon I learned that using the nail to push/slide/lift the stud in a sweeping blade motion to open/close the blade helps a lot. In this case, the flesh of the tip of the thumb is right against the blade and no part of the thumb shoud be on the pinnacle of the stud.
Also, disassembling the seb and then reassemble also make the new seb open /close smoother.