Whether you call it the late release, delayed release, down-cock or lag, holding your wrist hinge from the top of the backswing well into the downswing is a key to hitting long drives.
Unfortunately, plenty of amateur golfers drain this power source the instant they complete the backswing, casting the hands toward the ball and unhinging the wrists.
The late release should occur naturally as a result of proper lower- and upper-body sequencing. Whether you've got that down pat or not, here's an easy and effective way to increase your clubhead lag.
As you make a very slow practice downswing, keep your left thumb pointed up as you pull the arm downward toward the impact zone. To get a better feel for this motion, take the club to the top using just your left arm.
Hook your right forefinger around the shaft, then pull the club down slowly with the left arm, using the right finger to hold the angle between arm and shaft.
Keep the image of the left thumb pointing skyward and you'll enjoy longer, straighter shots.