Quick snow anchors can be devised from just about anything...including your body.
Most of you are probably familiar with Simon Yates and his infamous snow seat in Touching the Void. To say the least, that was an unusual situation.
The first way to deal with a potential shock-load in a snow seat is to add a snow anchor to back it up. This could be anything, but many climbers will simply use their ice axe. The belayer must then clip the climbing rope (which is tied to the climbers harness) to the snow anchor. Most will just make a clove-hitch with the rope and then slide the shaft of the ice axe down through the hitch. If the belayer has elected to use a hip belay, the tie-in must come off the same side of the climber's body as rope running to the climber, otherwise the load will twist the belayer uncomfortably.
The second way to deal with this is by using a dynamic belay. In other words, when the climber falls, allow the rope to run through the belay device for a short period of time, slowly breaking it and bringing it to a stop. This allows the snow seat -- and you -- a much smaller shock. There are clearly some problems with this technique and it cannot be used in every situation. The dynamic belay is only truly useful on steep snow climbs where there is little danger of a falling climber hitting something.
If we learned one thing from the Simon Yates in Touching the Void, it's that snow seats are an excellent option in terrain where you do not anticipate a need to escape the belay. If there is anything suspect going on, it's important to build a bombproof SERENE/ERNEST anchor.
When used properly, snow seats and dynamic belays can save a great deal of time...and as we all know, speed in the mountains is safety...
--Jason D. Martin