Now an entry into Fortnightly Topic Challenge #41: Short and Sweet
A man enters a room and presses a button. Within seconds he loses 20 lbs. How did he lose the weight?
an elevator and pressed the down button. The downward acceleration from the elevator reduces his apparent weight.
Since it's tagged 'lateral-thinking', an alternative that blatantly ignores the question's title is that:
The man is in the UK, and he just pressed 'OK' button to confirm a £20 purchase
He presses the button to start a machine. It catastrophically fails within seconds and takes off the lower part of his leg, which weighed 20 lbs.
For an answer which exactly fits the question and the OP's clarifications...
He is wearing a diver's weight belt with a quick-release catch.
Holding a 20 lbs of iron. The button activates a very powerful magnet.
In a room with some sort of anti-gravity (or, rather, gravity manipulating) feature. Just something that would slightly adjust the gravitational constant to make you lighter.
A somewhat less morbid variant of an answer that's been posted a few times:
He has a prosthetic leg that weighs 20 pounds, and the button releases its clasp.
The man previously lost 20lbs and without knowing this he came to check his weight on a weight machine and pressed button to check his weight apparently in seconds the result popped up showing he lost 20lbs, looks very natural and correct answer ! dont ask me exactly how he lost 20 lbs :-)
I know this already has a correct answer, but I'd like to offer an alternative:
The man works as a stocker and is holding 20lbs worth of cardboard. He enters the back room where he puts the cardboard in a baler and crushes it with the push of a button.
He walked into a Star Trek transporter (or similar) and transported himself to a planet where gravity was less strong.
... triggers a guillotine which chops a leg off (apparently ~20lb for an adult male)
It was an air lock of a submarine and filled the chamber with water, the buoyancy reduces his weight by 20 lbs.
The button published a new law that vital organs are owned by the government rather than the individual. He is no longer the legal owner of that weight.
The man was head of the International Committee for Weights and Measures and the button was publishing a new heavier definition of the pound.
Simple, the man pressed
the buzzer on his door and let in a cleptomaniac bodybuilder who proceeded to steal his 20lb dumbbell. He lost his weight because he's too trusting of other people.
Upon pressing the button,
The entire room starts to accelerate downwards (for example, he is inside the lift) at an acceleration of a
Given that his original weight is Mg - Ma = 20lbs, we calculate the a given his mass M.
My first thought was:
He entered an elevator and pushed a button, causing the elevator to go down. The acceleration downwards is credited with making the man 20 pounds lighter, if only for a brief moment.
Then I thought of this:
If "lbs" stands for British pounds, then the man could have been at a gambling casino. He could have bet twenty pounds on red in a game of electronic roulette. When he pushed a button, the roulette ball landed on black, causing the man to lose his 20 pounds.
Thinking about it some more, I thought:
If the twenty pounds are truly weight, then the man could have simply been separated from something on his person. This could be an arm or a leg (separated by, oh, let's say a button-activated-spring-loaded-sawblade), or just something he was carrying. Maybe he was a delivery man delivering a 20 lb. package that was ordered on-line. He rings the doorbell, then within seconds a spring-loaded-saw... I mean, a grateful customer takes the 20 lb. package from the delivery man.
But then I thought some more, and I came up with:
Maybe he bought a collection of Richard Simmons' "Sweatin' to the Oldies" videos (I've seen some for around that price). I can't really explain how he'd lose so much weight "within seconds," unless I reinterpret "seconds" to mean "going back for seconds" (often heard when eating a meal). Maybe he liked the videos so much, he exercised a second time, or managed to lose enough weight despite having more portions of food.
He entered a vertical wind tunnel which got up to speed slowly as he stood there after pressing the button
He enters lift, presses the button downwards. Psuedo acceleration upwards works here against gravity and weight measured is less now.
Let M be his mass and mg be weight before (outside lift)
Losing 20 lbs-
Pseudo acceleration is (Mg-20)/M unit length per square unit time