Two hands, slowly, with the edge pointed away from you. Pinch the blade spine between finger and thumb and rotate it open rather than flicking a stiff thumb stud. Then break the knife in properly rather than fighting it every time.
You apply increasing force against resistance until the resistance suddenly gives. The blade then accelerates far faster than intended, and your thumb is somewhere in the arc. That is the classic new-knife injury.
The fix is to remove the sudden-release element entirely by rotating the blade under control.
Hold the handle with the edge facing away from your body. Take the blade spine between the finger and thumb of your other hand and rotate it open. Never grip anywhere near the edge, and never place a thumb in the closing arc.
On a flipper, pull the tab firmly and deliberately rather than half-heartedly. A weak pull is what produces a blade that opens partway and stops.
Most new folders loosen up within a week or two of use, helped by a drop of oil and a few hundred cycles. See breaking in a stiff new folder.
If it is still stiff after that, adjust the pivot rather than continuing to force it.