Question: Why do beach volleyball players use hand signals?
About the article "Beach volleyball", section "Block signals".

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Beach volleyball players use hand signals to indicate the type of block they intend to make.
Block signals are made behind the back to hide them from the opposing team.
They are usually given with both hands by the serving player's partner prior to the serve, with each hand referring to the type of block that should be put up against an attack from the corresponding opponent.
A player may also "wiggle" or "flash" one block signal to indicate which opponent to serve to.
If the server is the designated blocker, he or she may run up to the net to block after serving.
Otherwise, the signaling player will perform the block.
Block signals may also be given during a rally while the opposing team is preparing their attack.
Footvolley combines field rules that are based on those of beach volleyball with ball-touch rules taken from Association football.
Essentially footvolley is beach volleyball except players are not allowed to use their hands and a football replaces the volleyball. 
In Mus, passing gesture signs indicating which cards you have to your team mate is perfectly legal, being a decisive strategic factor.
However, these signals can only be the ones specified in the rules as explained below, any non-standard signal is not allowed and would result in disqualification if proved.
Knowing your partner's cards and letting him know yours results in a more effective play and allows a more accurate evaluation of the team's chances to win each bet, however if the rival players see you or your partner while passing a signal, and gets to know your or his cards, your chances will be seriously compromised.
If you detect a rival's signal and the rival doesn't realize you know his play (that should require not to inform your partner about the signal you blew up, if he didn't see it) you'll become able to find a weak spot in his cards to counter-attack reversing the hunter-prey role or at least avoid his strong-point beads.
There are many commonly accepted signals ("keinuak") allowed in the game.
You can use any of them to indicate to your team mate the hand you are holding (while trying to hide the transmission of the signal from your opponents).
The signals have a fixed meaning and it is against the rules to use other signals or use false signals (by signalling a hand you do not hold).
The "keinuak" are: *Two Kings: biting the centre of the bottom lip *Three Kings: biting one side of the bottom lip *Two Aces: poking out the tongue *Three Aces: poking out the tongue to one side *Single Pair (pares): tilting the head to one side *Three-of-a-kind (Medias): pursing lips to one side of the mouth *Two-pairs (Duples): raising the eyebrows *31 (La una, i.e.
31-point Juego): winking *30 (30 puntos): lifting both shoulders *29: (29 puntos): lifting the right shoulder *28: (28 puntos): lifting the left shoulder *I have nothing (Ciego, blind).
This indicates a bad hand: closing the eyes *Royal 31 (31 real, see above): touching the earlobe Not all of these signals are accepted in all variations and there might be other signals in use.
It is always good to clarify the signals in use when playing for the first time.
Players have a wide variety of approaches to signal use, some players make a great number of signals, as soon as they have the chance, some players seldom or never make signals.
On the other hand, some players show a honed skill in catching the rival's signals whereas other players do not even make too much an effort to disrupt the opposing couple's signals; some would stare continuously and overtly to the rivals' faces for intercepting signals and others would pretend to get distracted or to be not too interested in catching rival's signals in order to make them overconfident and thus catch subsequent signals. 
This is a newer variation of beach volleyball.
As beach volleyball took volleyball outdoors, indoor sand volleyball takes beach volleyball indoors.
In the United States, a growing number of colleges are now considering switching from hard court indoor volleyball to sand court indoor volleyball.
The biggest reason for the possible change is the reduced rate of injury of players.
Secondary reasons are: 1) bad weather doesn't cancel play, something that commonly happens with beach volleyball; 2) it is thought to make the game more appealing to spectators since sand courts do not require players to wear knee pads or shoes.
Indoor sand volleyball teams vary from two to six members, college teams having six.
Normally, rather than using a purpose-built hall, an indoor basketball court is converted.
A protective tarpaulin covers the floor of the basketball court and "soft" sand is laid a foot deep over it.
The boundaries are commonly marked off with lines in the sand.
However, a recent innovation uses colored lasers that illuminate the lines in the sand.
In some venues, there exist sand courts that are used as usual during the spring, summer, and fall months, but during the winter months, a large tent (usually dome-shaped) is erected over the courts. 
 Mikasa makes the official balls of the Fédération Internationale de Volleyball (beach and indoor).
Molten makes the official ball of USA Volleyball (indoor).
Wilson makes the official ball of the Association of Volleyball Professionals (beach).