I would be surprised if more than a handful of the annual slide ascenders are rock climbers with enough training to A) Shout "Rock!" and B) React instantly. Can you enlighten us as to what that instant or trained reaction should be?
A) Whenever I have been in a group of hikers about to ascend a slide, someone (often me) instructs them in the proper calls and guidelines.
B) Reactions vary with the situation:
* If there is something to hide behind, hide behind it.
* If you are in an exposed position, hug the rock--make yourself as "small" as possible.
* Hold your pack above you to absorb the impact.
* If it is coming from high enough above, track it so you can try to dodge it if it comes toward you.
Each time a rock bounces, it can head off in an unpredictable direction.
I have seen rocks falling down a large face (~4k ft high). From a distance all you can see is puffs of dust at each impact point, up to hundreds of feet apart. Such a rock could skip over you or just as easily kill you.
Some other techniques to reduce the risk:
* Stay out of other climbers fall lines. (The fall line is the route that a rock would likely fall--typically straight down the slope.) Thus it can also be described as avoid getting directly below another climber.
* Avoid getting directly above another climber (this puts him in your fall line).
* Stay close together so that the rocks don't have a chance to gain much speed before they pass any other climbers.
* Learn not to knock rocks down. Pick your feet up vertically and carefully and put them down vertically and carefully--scuffing or normal walking sends showers of rock down. (Trained rock climbers can often pass over loose rock without knocking a grain of sand down while hikers passing over the same spot often send down a massive shower.)
* Wear a helmet.
* If you are using a rope, try to manage it so that it doesn't knock rocks down.
* Choose routes that have low risk of rockfall.
EDIT: thought of a couple more techniques:
* Wait until the party above clears off the climb
* On some alpine routes, the morning sun melts the ice and causes rockfall. Thus some routes are safest between ~midnight and ~dawn. You may also want a clear sky and cold night to freeze the ice and "glue" the mountain together.
Doug