At its core, Flail is about moving a green stick figure from the starting point to the goal, which is represented by a floor enshrouded in a beam of red light. Initially, the level designs are simple enough that you can accomplish this by simply running forward and jumping, but as the game introduces more play mechanics, it also begins to take the gloves off.
The core play mechanic of Flail, besides jumping, is "flight" - press the jump button, and you jump. Press and hold the fly button, and you immediately freeze in place, regardless of whether you're standing still or exactly 1 pixel above a pit of spikes. At this point, you have about half a second to hold one of the eight cardinal directions. After that half-second, you are violently shot in whatever direction you're holding at the time. This creates a lot of interesting new ways to move around, but you can't do it indefinitely - you have to land on solid ground before being able to fly again.
Besides the fly button, there's also a very simple wall-climbing mechanic. If you touch a wall right below its lip (assuming that it has one), you'll hang on. Any lower than that, though, and you'll fall, presumably to somewhere extremely unpleasant.
This is where Thorson starts messing with the rules a little bit, of course. There's an anti-gravity item, which immediately causes gravity to reverse after you touch it. There are levels that require you to flip gravity several times in wonderfully brain-bending ways, with goal platforms suspended upside down, sometimes directly above the starting point.
There's an item which lets you fly again after picking it up, allowing for stage designs that require you to fly carefully through a maze of razorblades suspended in mid-air while never touching the ground once.
There are also yellow, green and blue "fields," which are basically just different color tiles drawn over the backgrounds of certain stages. Green fields cause you to slow down when passing through them, yellow fields prevent you from flying, and blue fields cause gravity to reverse if you press the fly button when inside of one.
The ramifications for the last one are brutal. There's a stage in zone 5 (out of 8!) that requires you to carefully fly through another maze of floating razorblades, like several previous stages. The difference is that you also have to reverse gravity when flying through the tiny squares of blue scattered throughout, nearly flying off of the top of the stage at one point. And this is only a little more than halfway through the game, here! I'm trembling in fear at the thought of what future levels are going to look like.
There's also a level editor, which you unlock after clearing the first couple of zones - it lets you put stages together without any restrictions that I can see, other than that you can't compile stages into groups the way the actual game does. There's certainly a lot of design space to play around with, though, and I'm really looking forward to seeing what people come up with.
According to Flail's in-game statistics page, I've played the game for a little over an hour so far, and have died 474 times. That says it all, right there.
Download Flail from Matt Thorson's games page.