While the closed bridge can be used in about 90% of your shots, there are certain shots (i.e., shots close to the rail, stretch shots, shooting over an object ball) in which an open bridge is far more effective. Stretch shots are best served by the Five-Finger Open Bridge. The Open Rail Bridge, V-Bridge, and Over-a-Ball Bridge are extensions of this bridge, designed to meet the demands of particular shots. Each of these bridges should be practiced and mastered, to complement your overall game.
The Five-Finger Open Bridge is relatively easy to form. (Practicing basic shots with this bridge may make it easier to become comfortable with stretch shots, as well as the other three bridges.) Begin by putting your hand flat (palm down) on the table. Extend your fingers and raise your knuckles, to form a mini hand "tent." Nestle your thumb next to the knuckle on your index finger, without raising your thumb too high. Be sure to keep the heel of your hand on the table, to ensure stability and support.
Spread the remaining four fingers. The channel for the cue will be in the rounded "V" formed by the raising of the thumb. Be sure that the index finger does not face the cue ball. Move it slightly to the right (for right-handed players), to allow the cue to move freely. Since you're aiming near the center of the cue ball (and, of course, using a level stroke), the height of the channel should be roughly half that of the cue ball (about 1-1/8 inches off the table). This bridge should be used any time you're forced to stretch so far across the table, that you can't form a closed bridge without strain.