This is a great exercise to increase your lockout strength and is a good variation on the standard rack lockout.
To do this exercise, suspend a barbell in chains. You can do this from any height, but for lockout strength, we generally use 3-5" from lockout.
Using a close grip (which will vary from lifter to lifter, but is generally described as having your index finger just outside the knurling) press the bar from the chains. Hold this position for a few seconds and return to the chains.
You may want to place the safety hooks just below the barbell for some added security.