Yes when you hit the ball and then the ground (if an iron or hybrid), the clubhead will slow down, but then it speeds up again before gradually slowing down to the finish. Some people might feel this when they come in to steep on a shot and they get that jarring feeling through there arms, were they feel like they momentarily stopped, then they continue forward.
Also, why would it make sense to EVER decelerate on a short game shot before impact? Why wouldn't you just shorten your swing, open the face, hit a more lofted club, etc.?
What if the shot doesn't need more loft? I've played tons of short game shots with varied swing speed. Sometimes i need a short soft shot. I've opened the clubface and basically flipped the clubhead slowly through the ball and it just pops up in the air and lands like 3 feet infront of me. Believe me, you get on fast greens above the hole, short sided. Its a dangerous shot, but it works very well. Then there's times were i have to hit a low runner chip up a very steep green, there i might take a lower lofted club and play a very aggressive strike on the ball. Acceleration has nothing to do with contact.
If I even think for a second "decelerate,"
Here's the thing, you shouldn't be thinking either accelerate or Decelerate, it should all just happen if you have everything in the right motion. There might be a select few people who need to think accelerate through the ball, to get the proper motion for them, but it might not be what's happening at all in the golf swing.
isn't it a disruption of your natural motion to slow up before you make contact?
This is the same argument in reverse. If you say its disruptive to slow down before contact, then the opposite must be true for other people as well. It is disruptive for some people to try to accelerate through the ball. If this line of thought helps your swing, have at it, but don't make it advice or a tip for other's. But i am not making it a swing thought. If the golf swing is done right, then you will have maximum speed at impact, or slightly before impact. Meaning, you naturally slow down with out thinking about it. Think about it this way, from the top of the backswing to impact is probably 0.4-0.2 seconds long. Now when are you thinking accelerate. Is it at the top of the backswing, or just before impact? If its any were during the downswing it would be impossible to react fast enough to make a change. Average reaction time is about 0.22-0.25 seconds. That means, unless your thinking before you make your downswing, its probably to late to react. Once that downswing starts, your toast, what ever happens is going to happen. This is were feeling comes in. If you want to hit a soft wedge shot, you think about the shot before hand, changing your whole swing before you even start it.
A good example of this would be Tiger Woods at the US Open, when he practices for his big cut shot, he makes exagerated movements in preshot routine. He's rewiring his swing for that shot by making a movement that is exagerated. It would be impossible for him to adjust it once the downswing starts.