The mechanical bridge (known as the rest in the UK) is a device used in place of the pool players hand, for shots where it is difficult or not possible to place the hand behind the cue ball. All a mechanical bridge is, is a stick slightly shorter than a cue, with a shape on the end, suitable for resting the cue on
There are different styles of mechanical bridge, the most simple are an X shape, although the ones with a crown shape are more popular as these give more flexibility.
The mechanical bridge is less flexible and gives less control than making a bridge with your hands, and so it should only be used when absolutely necessary.
To use the mechanical bridge place the shape a few inches back from the cue ball on the table. Keep the other end of the bridge in your spare hand and angle the stick so that you can keep the cue running in line with where you want the cue ball to go. If you want to steepen the angle you strike the cue ball to increase back spin slide the rest nearer the cue, move it away to lessen the angle.
You will not be able to adopt a normal stance whilst using the bridge, but remember to keep your feet wide enough to support balance, and keep your head in line with the cue.
Remember, it is a foul to touch any of the balls with the mechanical bridge, so take care when setting up the shot, and make sure that if planning a rebound, that you will have time to remove the mechanical bridge and cue from the table