What should a player do in blitz if he knocks a piece on to the floor after pressing the clock? This question is prompted by an incident in today's World Blitz championship in Moscow as described and shown in this tweet.

What happened was that Nakamura made a move, pressed the clock, on the way back from pressing the clock his hand caught his rook and knocked it off the board about 10 cm. He immediately picked up the rook and replaced it. This maybe took half a second. 

His opponent, Sergei Zhigalko, thought for about 12 seconds before having a long look around, presumably to see if an arbiter was watching the game. Then after another 2 or 3 seconds he gestures over to the arbiter to call him to the board and indicates something with his hand. Finally 22 seconds after the incident he reaches out to pause the clocks. There then follows a long discussion between the players and the arbiters which suggests that Zhigalko was trying to claim an illegal move before the game continues with no action taken.

Nakamura acted perfectly correctly here IMHO, but what should he have done differently if instead of displacing the piece a few inches he had knocked it onto the floor requiring several seconds to retrieve and replace the piece?
The relevant section from the rules would be Article 7.4.1:


  If a player displaces one or more pieces, he shall re-establish the
  correct position in his own time.


The rules do not specify a certain procedure, but what I have seen happening in practice is: Player A knocks a piece over on his opponent's time. Player B is now allowed to press the clock to make sure he does not lose time. After the correct position has been reestablished, player A will press the clock again so player B has the move. I certainly don't feel the rules mandate that the arbiter needs to be summoned in this case.

If the displacement caused an unusual disturbance or the affected player is very low on time (&lt;10sec), as an arbiter I would consider applying Article 12.5 regarding external disturbances and add something in the range of 20-30 seconds to the clock of the affected player.

The following objections could be raised, although I feel they are rather academic in nature:


It would be in the spirit of the rules if the person displacing the pieces would stop their opponent's clock and start his own again. However, I have never seen this happening and it may well be inadvisable to do that just for practical reasons, depending on the circumstances.
If the game is played with increment, the additional two clock presses add time to the players' clocks. Therefore, the clock settings (times and move counter) would be incorrect after such an incident. However, at least in the German-speaking chess community, Article 6.10.2 is generally considered to only apply in cases where there is a non-negligible time difference. As a rule of thumb, it should take an arbiter at most one minute to adjust clock settings. If the time differential to be corrected is smaller, the arbiter should not interfere. Additionally, the move counter of the clock is not a reliable indicator of the number of moves played.


In the comments, Article 7.5.3 has been mentioned:


  If the player presses the clock without making a move, it shall be considered and penalized as if an illegal move.


This article has been added fairly recently to make sure a player can be penalized for pressing the clock for no reason. It should not be applied if the player enforces the rules.

Making a move and then expecting to correct the position of a displaced piece defeats the purpose of the rule: The distraction by the missing piece is immediately affecting the other player.