A snatch block is a single, double or even triple sheave block which can be quickly hung on an existing set of falls, the lines extending from an already rigged block. The snatch block is hinged on one or both sides, is fitted with a locking mechanism on one or both sides and opens at the swallow (the hole that the rope reeves through) so the block can be opened and clamped onto a line without reeving the line through the block.
Open the locking mechanism that keeps the cheeks of the snatch block closed, then open the cheeks of the snatch block. Keep the snatch block close at hand but not underfoot.
Attach the snatch block to the load to be moved.
Move the two-sheave standing block and lifting device over the weight to be moved. The standing block is the block to which power is applied, not the block which will move as it lifts the load. Spread the falls--the lifting lines--already rigged in the standing block so that they will not tangle.
Pay out sufficient line through the standing block to all the falls to reach the snatch block. Set the sheaves of the snatch block onto the falls, without twisting the falls, so there's a bight of line around each sheave in the snatch block.
Close and lock the cheeks of the snatch block.
The sheaves of a conventional block are larger than those of a snatch block; this means that you must use a larger snatch block than the conventional block used for the standing block in the rig. A snatch block should be chosen on the basis of its safe working load, not the size of its sheaves.
Rigging can be extremely dangerous, and appropriate caution and, if necessary, training is advised.