found this on the net but must admit i don't really see the point in it

Crossbooking is where you and another player book each others wins or loses.
Say you are crossbooking me. We write down our figures, meaning the amount we
have in front of us. We then proceed to play. At the end of the session we
settle up. This is done by looking at our original figures and seeing how we
have done.
Say Party A won $1,000 and Party B lost $1,000. Party B would thus owe Party A
$2,000. $1,000 for the amount Party A won and another $1,000 for the amount
Party B lost.
This used to be done all the time back when, until others finally realized how
much this was a scam.
If Both Parties win, the person winning the most will have his win differential
matched by the other.
They have double and triple crossbooking also, making the stakes far higher.
The term means this: I will book your win and loss and you will do the same
with me. If I lose $2000, I will pay you an extra $2,000. If I win $2,000, you
will pay me an extra $2,000. The same applies to the other party.
Sometimes only one person is being booked. This is called booking a person and
only what he does in a game is considered. When you crossbook, both players
figures come into play.
The differentials between the stacks can be great, if player A happens to win
and player B loses. Basically it just doubles the difference on how big you are
playing if just plain booking a player. Crosbooking makes the stakes far
higher. Double or triple cross booking makes the stakes way higher. The reason
it is no longer common is because those crossbooking would fill the game up
with friends that would intentionally dump chips off to the person doing this,
thus making him a big winner.
Prices can be laid on crossbooking to kind of equalize things. This is done by
a player stating he will lay odds on the outcome, say lay $1.50 to $1.00. This
means if player A was to win $1,000, he would be rewarded with $1,500 for his
win.