How do you call an exercise that consists in holding the dip bars with the arms straight (during the whole exercise) and contracting the abs? What is it good for?
If you're just engaging the abs, that's "bracing".
Abdominal bracing is the second way to strengthen your stomach with simple tightening maneuvers. To do the move, imagine preparing your stomach muscles for a punch in the gut. You’d automatically contract and stiffen your midrift to brace for the jolt, right? That’s abdominal bracing, which is a key component of plank and its many variations, as well as push-ups. Abdominal bracing activates all three layers of the entire abdominal wall, causing them to bind together. It works both the deep and superficial muscles.
More likely, given you're using the bars, is that you're doing "hollow body", which is the same technique, except you're bringing your head and legs in as well, and sort of "sucking in" the gut. This is more useful, because it promotes a whole-body movement and static strength, and can be done in a larger variety of positions. You can see a video of this being done on the parallel bars here which sounds like what you're describing.