You got me thinking about this and up until now I never thought about before. So I looked up the rule:
"Rule 4-1-3-b (Exception) clarifies that the holder on a kick play may have his knee on the ground, provided a kicker is lined up in kicking position at the snap.
The knee can actually raise off the ground and still return immediately to kick the extra point or field goal, but once you catch the ball with your knee on the ground and raise up you are no longer considered the holder. Therefore the next time your knee hits the ground with the ball still in possession you are considered down."