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Some useful information from Supertraining and Triphasic Training about isometrics:

-In order to carry over to other movements, the joint angle you are training isometrically has to be within 5 degrees of the joint angle of the movement you want to improve.

-Not recommended close to important competition: might decrease coordination and speed in the short term

-Excellent for absolute strength - if a certain stage of a movement requires a maximal contraction isometric training is great for improving that stage. It improves rate coding, allowing you to generate consistently high levels of tension, rather than intermittent bursts.

-There are several types of isometrics:
--Resisted load: like a paused squat or paused bench - it is recommended that the eccentric and concentric portions of this movement be as fast as possible. Drop like a brick, come to a dead stop (not a gradual slowdown) and explode back up. Good to start workouts with this to recruit more MU's for the rest of the workout. Not very CNS taxing, especially if done as RFD work.
--Immovable Resistance Isos: these are like wall pushes. These are very CNS intensive, equivalent to Max effort work. Very time efficient, should take no more than 10 mins. Good for building strength without size. There are 2 subcategories:
---Absolute iso: Gradually build up to max contraction and try to maintain it. The whole contraction should be no longer than 8s w/ 10s-30s rests between reps. This is what you would use to improve a slow max effort lift.
---Explosive iso: You are trying to achieve a powerful (70-80%) contraction as fast as possible. This is what you would use for training for dynamic movements.