There are many decent ball games out there. There are many not-so-decent ball games out there.
Spank, however, is in a league of its own.

The primary objective in Spank is to spank balls past your opponent, into the goal. It's a glorified handball penalty shootout, basically, although if you told that to a professional, they'd likely spank you in the face for blasphemy.
To spank, you start behind a line. You throw the ball over the line, into the air, promptly running after it. As soon as the ball bounces, you literally spank the ball towards the goal, aiming to get it past the defender.

Both players take it turns to be spanker/defender. The spanker gets to play a RUN of 10 SPANKS each, before switching places with the defender. Whoever gets the most points wins a GAME. First to four games wins the MATCH.
The subtlety of the game lies in the scoring. If a spank winds up in the goal, it scores 1 point. If the ball does not end up in the goal, and the defender has made a touch or a catch, the spank scores 0 points. If the ball misses completely, or if it hits the posts or crossbar, with no touch from the defender, the spank scores -1 point. The dilemma of Spank lies solely in the hands of the defender, who has to make a quick, informed decision whether or not to go for a save.
Another far more pressing dilemma is your choice of clothing. Depending on the temperature, you might like to remain entirely topless whilst out spanking, simultaneously soaking up the sun and minimising the surface-sweat-to-clothing contact ratio. The best compromise is to wear the closest thing you've got to a tank top, thus allowing freedom of movement and maximising the sport's fashionable potential. David Beckham, eat your sodding heart out!

Once you've got to grips with the basics, you can start to consider movement allowances, rebounds, fouls, fetches, tiebreakers, shitfetching and advanced scoring. These wacky nuances make Spank what it is: bloody excellent.
AREA OF PLAY
One serving line, 32-40ft, parallel to the goal (4½ft high, 6½ft wide), separation distance 40ft. A 'protection zone' of 48ft in width and 12ft in length is marked out behind the goal. The main playing area is of unlimited width.
EQUIPMENT
Sponge or softball, slightly smaller than the size of a standard football. Available from any good branch of Tesco.
METHOD OF ATTACK
Starting from behind the line, the attack is carried out as follows: Throw, Run, Bounce, Spank. The run (over the line) may not precede the throw, and the spank may not precede the bounce, on pain of a foul being awarded. The spanker should aim to drive the ball into the goal. If the defender saves the spank and the ball remains airborne, the spanker can play one or more rebound spanks until the ball begins to roll, or is stopped or caught by the defender.
METHOD OF DEFENSE
The defender starts at the goal line and cannot leave it until the spanker's initial throw has bounced. Leaving it incurs a foul and allows a repeat shot to the spanker. After the bounce, the defender is allowed to attempt to save the ball, either by intercepting it before or after spanks and rebounds, or by catching it before its entry into the goal.
SCORING A SPANK
If the ball enters the goal after the defender fails to save it, the spank scores +1/1.
If the ball fails to enter the goal and the defender touches the ball at any stage during the attack, the spank scores 0/1.
If the ball fails to enter the goal and the defender does not get a touch to the ball at any stage during the attack, the spank scores -1/1.
SCORING FOULS
If the spanker fouls during their attack, the spank scores an automatic -1/1.
If the defender makes two consecutive running fouls or fetch fouls (see below) at the beginning of an attack, the spank scores an automatic +1/1.
RUNS
A spanker gets a 'run' of 10 attacks in which to accumulate a maximum score. Scoring is aggregate and is documented between attacks (e.g. 0/1, 0/2, 1/3, 2/4, 2/5, 2/6, 3/7, 2/8, 1/9, 1/10). After the spanker's run is complete, the spanker and defender switch places and play another run. The higher scorer at the end of both runs wins a 'game'; a tie leads to a tiebreaker of 2 attacks each until a winner is found.
FETCHES
The spanker has the option of calling a 'fetch' under certain special circumstances: if it is called, the defender must relocate to the position where the ball lies following the previous attack, throwing the ball to the spanker who may immediately begin the attack without waiting for the defender to return to the goal line. This gives the spanker a hugely increased chance of scoring a +1/1, as the defender will not be ready to make the save. The conditions for calling a fetch are:
- The previous attack must have scored a 0/1.
- The previous attack must have consisted of a single touch to the ball by the defender.
- The ball must be lying in front of the serving line at its point of rest.
- The ball must be moved back to the nearest edge of the protection zone if it travels beyond it.
A spanker is allowed two fetches during the course of one particular run: both, one or neither of these may be deployed at the spanker's discretion, but they do not carry over between runs, and cannot be deployed during tiebreakers.
SCORING A MATCH
After each game, the player who got to play the second run in the previous game gets to play the first run in the next, and vice versa. The match continues until one player wins 4 games, or until both players tie 3-3, at which point a final tiebreaker of 2 attacks each is played to decide the overall winner. Matches average 20-40 minutes in duration.
TERMINOLOGY
'Blank' or 'break' - 0/10.
'All to play for' - when the 10th shot in the run (or 2nd shot in the tiebreaker) can send the final outcome of that particular game either way (win for +1/1, lose for -1/1).
'Too right all to bloody well play for' - an 'all to play for' that can decide the final outcome of the entire match.
'Shitfetch' - scoring -1/1 in spite of an excellent fetch throw position.
'Mar' - two shitfetches occurring in one run.
SCORING TERMINOLOGY
Scoring is declared in the form of three or four consecutive numbers.
"Three, seven, one" means a score of 3/7, with 1 fetch to hand.
"Three, seven, four, two" means a score of 3/7, with a score of 4 to beat, with 2 fetches to hand.
"Three, seven, four to beat, two fetches" can also be used to elaborate on the same.
When both fetches are deployed, "zero fetches" can be omitted from the declaration.
You can download one right here. This is what they look like when they're filled in:

If you're using the card during a match, you can use plus signs instead of greens and minus signs instead of reds. You can also circle a square to indicate a fetch, as opposed to an ƒ (ALT+0131).
Headbutting is disallowed, although this is subject to review should pop-cultural changes come into effect.
(Thanks to Sister Katy for the pictures. "Pie Iesu domine, dona eis requiem.")