This explains the secret of the bowling ball trick. Direct all of a vacuum's power onto one surface area and you can do amazing things. You have seen the commercials, right?

A vacuum's ability to lift is a valid measurement. In fact, it is called vacuum, or water lift. See how strong a vacuum can be by simply swallowing the air out of a two liter plastic soda bottle. That is actually about all it takes to lift a bowling ball!

The suction capacity of a vacuum  is not the key to effective vacuuming. The real key is how much air the vacuum moves and how fast it moves it (see CFM below). Water lift becomes somewhat important when the air opening size is smaller - for example in air driven brushes, because the strength of the suction keeps the air turbine spinning when tension increases.

Water lift measurements will fluctuate based on the actual amount of voltage the motor is receiving, the altitude, air temperature, and the barometric pressure.

CFM (Air Flow)
Cubic Feet (of air) per Minute. (Usually give at 2" air opening).

Wonder why that pebble won't vacuum up but stays rattling in the end of your vacuum wand? Basically, without sufficient air moving around it, it's going no where. Even a feather won't budge without any air moving past it. Therefore, CFM (air flow) is crucial for deep carpet cleaning. Some vacuums may be able to lift bowling balls, but the cleanest homes have  vacuums (ex: central vac's) with tons of CFM. CFM becomes more important as the air opening size gets larger.