The orginial term lathe is where a person is hacking into wood that is pivoted on two ends with his chisel. In the sexually degrading version, the chisel is a mans penis and rather than hacking into it he is wanking onto the female. The pivots on two ends are the "traditional spitroast".
There are a few rules assosciated with a lathe:
1. No crossing swords.
2. When eye contact is made a high five must be given.
3. Which dude will recieve the blowjob, which guy will fuck her doggy and which one(s) wank over top, must be prederteremined before the spitroast.
This is to avoid the complicated situations when all males only wish to get oral from a less than average female, or when all wish to fuck a rare 'honey' that can be conned into a lathe.
A typical lathe involves two males pivoting the female on the ends, with one wanking over top. However more males can chip in with the 'lathe' action by wanking over top as well. When this occurs typically they are on opposite sides, and if there are 3 or more males wanking, they can either alternate sides or have a rotation system.
Babe: Sure thing but there is more than two of you so howsabout we turn it into a lathe?
Hustler #2: Deal
Hustler #3: Sure
Hustler #1: Shotgun being the chisel in the lathe!