The first number stands for the amount of Nitrogen which contributes to foliage and root growth. The second number stands for the amount of Phosphorous, which promotes spiking and flowering, and the third number stands for the amount of Potash, which gives the plant vigor and helps it combat disease.
So it's the second number that's higher on fertilizers sold as "bloom boosters." The increased amount of Phosphorus stimulates the plant to produce flowers, but it doesn't make the flowers last any longer. And it won't "force" a plant to bloom if the plant isn't already getting ready to do that of its own accord. What it will do is is make the blooms larger, make buds develop faster and more profusely on a spike, and make the spikes generally longer and somewhat thicker.
We use it only once plants have already initiated their spikes or sheaths.
Hope that helped!