Thanks for the link Z.
Unfortunately we don't all have winches, so snatching is a bit more common than it should be when we're out on a trail.
I always try to emphasize to those standing around to clear the immediate area and move to a safe distance before we attempt snatching - and I was glad to see that most of the folks did this when we were in Las Cuevas last week.
Here's an excerpt from that link that Z gave us:
When to use it:
This technique should only be used as a last resort, as it is probably the most dangerous technique, and there have been some nasty, near fatal incidents. The pulling force that can be generated far exceeds that of any winch or conventional pulling - even if the recovery vehicle is on slippery ground.
Necessary Safety Precautions:
Only use proper robust recovery points on the vehicle. The lashing eyes on Land Rovers are not recovery points, and will rip off and become lethal projectiles!. Shackles used on the recovery points should be at least 4.75 ton rated bow shackles. Open the bow shackles half a turn to prevent them from binding under pressure.
In most instances, feed a bridle through the end of the strap, and attach the bridle to two recovery points. Do this on each vehicle, unless a recovery points has been specially fabricated to withstand these high forces.
Never connect two straps with shackles! If one of the straps breaks, the shackle becomes a lethal projectile. Rather use a light weight wooden stick (broom stick section) or even a rolled up newspaper. You can feed the two strap loops through each other, but under severe loads they will bind together, making it very difficult to undo them again. Some experts maintain that this way undue forces are exerted in the entwined loops.
A nylon rope (about 5 to 6m long) should be tied around the the snatch strap close to the loop at the recovery point and wrapped a couple of times around the strap, and to another secure point on each vehicle. Do this on both vehicles. These two nylon ropes would then absorb some of the energy from the resultant recovery point missile, should one break off. Also, drape a blanket, tarpaulin, or something heavy and flexible over the centre of the strap, to absorb some of the strap energy should something break.
If at all possible avoid snatch towing a small light vehicle (eg. 1400 Nissan) with a much heavier vehicle (eg a Defender), and visa versa.
Make sure that the attachment points (eg shackles) do not have any sharp edges that may cut the strap.
Make sure that the strap is not twisted.
Only use rated snatch straps.
Everyone, apart from the two drivers, should stand at least twice the strap length away, to avoid flying missiles should the strap or an attachment break.
The drivers in the two vehicles MUST wear seat belts and preferably crash helmets.
The driver of the recovery vehicle should have some robust protection between himself and the snatch strap, such as a tyre.
The driver of the stuck vehicle should have the bonnet raised as a shield.
Because the two drivers can't see each other, a director needs to stand outside the danger zone, in a position where he can be seen and heard by both drivers.
The director coordinates the snatch, during which both drivers release their clutch in the same gear (eg 2nd low).
When the bogged vehicle is successfully extracted, same vehicle hoots to let the recovery vehicle know that he is free. The recovery vehicle stops, and the now released vehicle carefully reduces the gap between the two vehicles until there is slack in the snatch strap.
Only when the director declares that it is safe, may other helpers approach and help remove the recovery gear.
So why is snatching so dangerous?
As mentioned in the beginning, a huge amount of energy is stored in the snatch strap. The energy stored is half the vehicle mass multiplied by the square of the vehicle speed. Therefore, the forces exerted at say 20km/hr, are 4 times those exerted at 10km/hr! Our desire is to use all this stored energy to pull the bogged vehicle out of its mud-hole. But what if it doesn't?
The main dangers in order of magnitude are:
The worst that can happen is that the recovery point (or indeed a chunk of chassis) of one of the vehicles tears off. This piece of metal will now be accelerated by the tons of tension stored in the strap, in the direction of the other vehicle. This piece of metal can easily achieve speeds in excess of 700km/hr, depending on size (see calculations later). This projectile can cut a swath of destruction right though the vehicle. The best you can hope for is that no living soul is in its way! This missile could also strike a solid part of the vehicle eg the winch, and bounce off at tremendous speed in an altered direction. This is why everyone, including the director (the two drivers excluded), should stand at least the extended length of the strap away.
If too much force is used at once, the bogged vehicle may come loose with the strap still having lots of stored energy to spare. This stored energy then makes your newly de-bogged vehicle accelerate faster than a Ferrari on steroids and literally go airborne only to come crashing down again (most likely onto the towing vehicle). Alternately, the recovery vehicle loses grip and is catapulted back into the bogged vehicle. The important thing is that you always start gently and gradually use more force at each attempt. But there are limitations, as the strap does not retract to its original length straight away. More on this later.
The snatch strap may break. This usually happens where it is in contact with the tow point, or where the webbing has come loose at the eyes. This results in a missile launch similar to when the tow point breaks except that this time only the rope is flying. Even the strap itself can be lethal. But you have protected the drivers with the open bonnet and tyre, and all the spectators are far away.