Here are the rules of Spit explained to the best of my ability. What is important to emphasis is the sheer pace involved in this card game. It is utterly frantic, when played by two relatively experienced players, it is also enormous fun. Players should be seated at opposite sides of a (preferably low) table, with a full deck of normal playing cards (minus the jokers) divided equally between them.
The object of the game to get rid of all your cards.
If you know how to deal a game of Solitaire then skip this paragraph, deal identically to Solitaire but with only five rows. Taking their unexamined cards players deal a row of five cards in front of them. The first should be face up and the other four face down (I usually start from the left, but I don’t suppose it really matters). Then each player deals four cards on top of the face down cards, again the first new card should be face up. So what should be dealt is: one pile with one face up card; one pile with a face up card on top of a face down card; three piles of two face down cards. Continue dealing in this manner, i.e. deal one face up and two face down onto the three remaining ‘blank’ piles, then one face up and one face down onto the two remaining ‘blank’ piles and finally on face up card onto the last pile.
This is what should be dealt: ‘0’ denotes face up ‘X’ denotes face down.
The cards are placed one on top of another, the below diagram is for descriptive purposes.
0 X X X X 1st time
0 X X X 2nd time
0 X X 3rd time
0 X 4th time
0 last time
Once finished each player places remaining cards above dealt cards and to the right so that the table looks like this:
     <-- opponents dealt cards
  <-- extra card pile
     <-- your dealt cards
The cards should be dealt as fast as possible to build up momentum for later in the round. Once dealt there is one other operation players are allowed to perform before the round starts proper. If two of the cards dealt are the same value it is allowed that they be moved on top of one another and the card below the moved card can be turned over. This may be done as many times as there are similar cards in different piles. If the moved card is the last in a pile, it is permissible to take a face down card from another pile (by lifting the face up card(s) first). Sometimes there will be much opportunity for movement here, other times none (this depends on how well shuffled the cards are and how long the game has been in progress for). The quickest player must at this stage wait for the other before continuing to the next stage (it is usual to taunt the slow-coach while waiting, in order that they go a quick as possible).
Now on the count of three both players turn over the first card on their reserve stack. This is where the fun begins. The trick is to be quick. Players can play on either card, not just their own. A valid play is a card one up or down from the card shown, e.g. if the card played from the reserve is ‘4’ (of any suite) then either player can place a 3 or a 5 (of any suite) from their on top. The counting is cyclical i.e. an Ace is one above a King. At this stage it should become apparent as to why similar cards are placed together. If for instance in the above example you had several ‘5’s and ‘6’s then if the other player cannot play on the 4 (or you are fast enough) you could play most or all of those cards. However once you play a ‘6’ your opponent may be able to play a ‘7’ and thus ruin your run. It is easy to develop technique once the rules have been mastered so I won’t digress much further.
Once both players have exhausted all their moves i.e. played all possible cards, turned over any revealed face down cards and ensured they have five rows (creating a new one if necessary as above), then there is a pause. Each player makes sure the other is ready to draw a new card, by asking or looking and raising an eyebrow enquiringly. Then after a count of “One... Two..Three...” another card is drawn from each reserve stack and the ordeal continues.
Once you have only five (sometimes this is played as four, agree before beginning) cards remaining in front of you, you can take them in your hand. If you have played all the cards in your rows then (but for one small detail) you have effectively won the round. The detail is this: you must shout “SPIT!” and place your hand on one of the piles.
     <-- opponents cards
 || ||  <-- reserve card piles at extremes, playing piles inside
<-- all your cards are gone
It would be advantageous to pay attention to the size of each pile, and perhaps even deliberately play more frequently into one. Pick the smaller pile, because once you do so the round ends, and the pile you pick is added to the remains of your reserve stack. Sometimes it is played so that once your opponent sees that you are about to finish, they can shout spit and grab a pile, regardless of their state of completion. This is a valid rule but some people consider it unfair. However the case will probably arise with equally good players where they will finish simultaneously and this is where speed is of the essence (be prepared for some finger collisions).
Once the second round has begun, play continues as above. Depending on how the previous round went the situation may arise where one player completes his reserve stack before playing all his cards. If this is the case then he must wait for his opponent to draw from her reserve stack and he must play on the other person’s pile until he finishes all his cards (he then would be wise to ‘spit’ on his own pile, as it has been played on less).
When one player has less than 15 cards at the end of a round then he should deal in the proper manner for as long as he can and then turn over the top card on any unfinished piles. Play as normal (with only one playing pile obviously) and when someone finishes they should ‘spit’ on the empty space where there should be another pile. Sometimes there is a rule that there must be a card on the table instead of his paying pile, to prevent the two players ‘spitting’ on two different parts of the empty table and claiming to have won. This isn’t absolutely necessary if both acknowledge a specific part of the table as the designated area. If the winner of the round has no cards left then he is the winner of the game.
     <-- opponents cards
 || X <-- ‘spit’ on the X
<-- all your cards are gone
This is a very long-winded explanation, which gives the impression of a very long a tedious game. It isn’t. A game including several rounds, may last anywhere between 2 and 10 minutes approx. The trick is to play as quickly as possible. This includes dealing at the beginning, it shouldn’t take longer than about 30 or 40 seconds to deal and sort your cards.
I hope I have described this adequately. If I have and you choose to try it out you will no doubt discover what an addictive game it is, perfect to waste 20 minutes or so on a rainy day.
I only remembered that this game is sometimes called speed after I had written this. There is a good writeup there and at speed card game too. There is some overlap, but some differences too. Silly me.