"The minimum velocity that a body must attain to escape a gravitational field completely"
so my inference for gravitational field is Bear Den or a trading range whereby once you leave it , it is for ever ("completely") so meaning permanently high prices...Or in other words: Blissful utopia....
The stock market, by default, is always in a bear market. This is why prices seem to decay faster than they do when they rise. In essence, prices can only move higher if there is more demand (liquidity) than the limited supply of the float that's available to be traded. If there is an imbalance in this velocity in which money is moving into the market, it then becomes easier and easier for prices to move in this desired direction with very little effort.
It's very much like the launching of the Space Shuttle...you need maximum (breadth) thrust to lift off the platform - the higher you go, the less fuel you'll need as gravity loses more and more of its influence (the bear market default), to the point where even the smallest of fuel bursts allow the craft to maintain its orbit while fighting this never ending pull of returning the craft to Earth.
The same can then be said of the market when it reaches the +500 level on the NYSE Ratio Adjusted McClellan Summation Index...it's the point where there has been enough energy (thrust) behind the trend to attain a high enough liquidity pool and prices continue to rise more easily because of this. There also times, however, where we can fail just as Challenger unfortunately did as they "throttled up" back in 1986, and you return to earth with unintended consequences in tow.
Here's another analogy I like to use...if you step on the gas pedal in car, you will continue to move forward at faster and faster rates of speed as long as you maintain your foot on the pedal. Once you decide you're going fast enough, you then take your foot off the pedal, but your forward motion continues as you decelerate slowly to the point of stopping. This deceleration would be kin to seeing less and less stocks in the SPX participating when you were at maximum speed, but the SPX itself, in total (the car), continues to move forward anyway until it finally "exhausts" itself of this forward motion created by your keeping your foot on the accelerator pedal.
Anyway, that's the concept. And remember, the great majority of the better indicators out there were developed by "rocket scientists" for they understood the beast that is the equity markets.