As Bozz said, this is using the units of the variables/quantities you are given to arrive at the answer. On the MCAT this can be a very poweful technique for a number of the chemistry and physics problems. Here's an example:
Often you'll get a passage that gives you some new formulas that apply to a specific situation. Let's say...an electron passing through a curved plate capacitor: You might be given some novel equations that deal with the velocity, energy, etc. of the electron. Then you'll get a question which says something like:
"Which of the following expressions describes the force that is applied to the electron?"
The answer choices will all be expressions containing variables and quantities that are in the passage. Now, there are two ways to attempt these types of problems:
1) Figure out how the formulas that were given in the passage (usually complicated) and the formulas that you know from your studying fit together and eventually simplify down to one of the answer choices. This usually requires some tedious algebra, and also understanding of the equations themselves.
This is the slow way.
2) What is funny about the MCAT is that questions like the one above, will often be structured so that
every single answer choice will have different units! 
Since the question asks for an expression of force, one of the answer choices must have the units of force (kg*m/s^2). If you realize this, you can easily exploit dimensional analysis to arrive at the correct answer.
You just take 20 seconds to check the units of each answer choice, and then answer that has the units of force must be the right answer. Et Voila! Done in under 30 seconds...
On the MCAT board, when I harp about learning to do problems "in the most efficient, quickest way possible, using the least amount of paperwork and math", this is the kind of thing I'm talking about.
