You may have heard recently that the number of people under correctional supervision in the U.S. has been steadily falling over the last few years–from 7.2 million in 2008, to 6.97 million in 2011. While that's true, it's also true that the decline is happening exclusuvely at the state and local levels. The federal prison system has only grown since 2008. Federal detention facilities are currently at 139 percent capacity, and, absent any reforms of federal mandatory minimum laws, are expected to grow indefinitely. Enter Sens. Rand Paul (R-Ky.) and Pat Leahy (D-Vt.), and the "Justice Safety Valve Act of 2013," which was introduced today.

Here's why this bill is important: A guy–let's call him Weldon–sells pot to a government informant, who notices that Weldon has a gun strapped to his ankle. The next time the informant buys pot from Weldon, he notices a gun in Weldon's car. When police move in to arrest Weldon, they find guns in his house. Weldon has never fired these guns, never used them to coerce anyone. He has, however, sold pot three times* while in possession of a firearm, so prosecutors charge Weldon with "multiple counts of possession of a gun during a drug trafficking offense." He is convicted. What do you think Weldon's sentence is? Ten years? Twenty years? Try 55 years–five for the first gun-related offense, and 25 for the second and third. That's the mandatory minimum federal sentence for Weldon's charges, meaning the judge who sentenced him could not sentence him to less time–only more.

Weldon Angelos is a real person, by the way, and the existence of a safety valve in 2004, the year he was sentenced, would've allowed the judge to sentence him to 18 years instead of 55 (that was the judge's preference). It would've meant Weldon, who was 24 at sentencing, would go free at age 42 instead of age 79. But because the federal system has mandatory minimums with no parole, Weldon will spend most of the rest of his life behind bars for selling several hundred dollars worth of pot while wearing a gun on his ankle.

The need for reform is all the more pressing when you consider how many other Weldons are behind bars at the federal level: 218,000 prisoners who doing time in a federal facility in 2011, and 48 percent were drug offenders.

So how does the safety valve work? Julie Stewart, president of Families Against Mandatory Minimums and the chief consult on the Paul-Leahy bill, explains:

Safety valves allow courts –in some circumstances –to sentence a person below the mandatory minimum if that sentence is too lengthy, unjust or unreasonable, or doesn't fit the offender or the crime. For example, a safety valve allows the court to avoid unreasonable outcomes, such as a first-time drug courier getting the same sentence as a major drug kingpin.

You can read more from Stewart (and Grover Norquist!) in a recent op-ed for The Hill. In anticipation of concerns from prosecutors and tough-on-crime types, the Paul-Leahy bill has pretty strict guidelines for determing when the safety valve can be used:

Judges are not required to use the safety valve.

Judges must base their below-minimum sentences on the criteria listed in 18 U.S.C. § 3553(a), then consider the sentencing guidelines, not merely substitute their own judgment for the dictates of Congress.

Offenders do not have a right to a sentence below the mandatory minimum.

If the judge gets the sentence wrong, the decision can be appealed and reversed.

Before judges apply the safety valve, the Justice Department has the right to argue that the mandatory minimum sentence is the correct one.

Judges must place their reasons for sentencing below the mandatory minimum on the record, in writing.

FAMM's report on the Paul-Leahy bill lays out the fiscal savings of implementing the safety valve, and includes more stories of people who received perverse sentences due to mandatory minimum sentencing. The bill isn't a panacea, but it is a long overdue step in the right direction.

*This post has been updated to reflect that Weldon Angelos' sentence was a product of him having sold pot three separate times while in possession of a firearm.