I think mostly everyone can agree just how awesome it is to complete a set of deadlifts. However, missing a repetition typically evokes a feeling that exists on the opposite end of the spectrum. The worst of it is when said miss occurs due to a weak grip – no one likes to successfully lift a heavy load of the ground, only to have it slip out of one's grasp.
If you pay attention, loss of grip in the deadlift doesn't always occur with both hands at once. It usually begins with the slackening of the weaker hand, which causes the dominant hand to grip even harder to compensate. However, this only serves to tire the stronger hand even quicker. Before long, the barbell rolls off the hands and collapses on to the ground.
Timed holds have always been used to great effect when it comes to improving grip strength; most employ this tactic via movements like farmer's walks. However, exercising greater specificity in movement selection tends to result in a better carry over, which is where the single-arm rack pull comes into play.
Location and equipment courtesy of TripleFit
While performing and holding a rack pull will certainly have some effect on training one's grip strength, doing so unilaterally allows you to focus on the weaker hand exclusively. Compared to farmer's walk variants (like the single-arm dumbbell/suitcase carry), the single-arm rack pull is more beneficial due to the resemblance shared between hand orientation. The relative mechanical disadvantage afforded by the pronated position of the hand also creates a greater training stimulus.
Set the rack hooks/supports so that the barbell rests above your knees. Grab the barbell in the centre (use the knurling if there is one) with your weaker hand and brace your body, creating tension in your core as you do. Do a controlled lift-off and hold the barbell in front of your thighs with your hand squeezed as tightly as possible. Hold for time before lowering it back down slowly to the starting position.
Elite powerlifter Blaine Sumner credits this exercise as being one of his favourite ways to train grip strength (his deadlift record is a monstrous 347 KG), so there is a high level of legitimacy associated with this movement. Sets of 5 to 10-second holds tends to work best and you can go for as many as 6 sets in a single session. Just take note: you will start to quiver and your grip might threaten to slacken as you get fatigued. Hang on like your life depends on it!