Pain
The twins are looking at the King from afar as they observed his symptoms. According to Sam, the pain will only get worse for the next few worse.
And they have to make a move only when the King tries to cut his hand off but fails, that is the best time for them to make their move as that would be the time the pain receptors reach the highest sensitivity.
Before the symptoms reached that point, Sam already arrived and took a look.
They had to wait for a good half-an-hour before they could make their move and this time, Sam didn't go himself into the field, he just flew upwards on the Sky as he looked down through the telescope.
The twins arrived in front of the King as he was about to chop his arm off, but the very little contact made him unable to continue. The small slash he left on it created a lot of pain and faint bleeding and he could only grit his teeth and endure as he took deep breaths.