To clarify I'll provide some examples of what I mean by "holding":
Say when you are doing a situp, you can hold the position part way through and then just stay there and it will work your muscles. Another example is holding a dumbbell horizontally, or doing a chin up so that your biceps are approximately horizontal and then just holding that position.
Is doing exercise like this bad? Why is it never recommended to do this over doing the entire motion? What are the advantages and disadvantages of each? Are there dangers of holding?