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In addition to placing straight ufc bets, some bookies allow you to place "proposition wagers". These lines can either be unrelated to who wins, such as whether a fight goes to a decision or not, or apply more stringent conditions on who wins, like that a given fighter wins by submission.
Here is some important information to keep in mind when dealing with proposition wagers.
If you're confident that a given fighter is likely to win a certain way, then a proposition line can often have a lot of value. For instance, Michael Bisping was -340 against Jorge Rivera, however the line on Bisping finishing the fight was +160. Given that Bisping was heavily favoured to win, and Rivera had never lost by decision, there was quite a bit of value on the proposition line. As another example, St. Pierre was -450 to win outright against Koscheck, but was -200 to win either inside the distance, or give up less than two rounds. Given that St. Pierre had not lost a round in his past six fights, that line had a lot of value.
The obvious disadvantage on betting on propositions is that if you bet on a given fighter to win by decision, and that fighter wins inside the distance, you lose your bet. However a less obvious disadvantage is that when you bet on a given fighter to win by decision or inside the distance, and the fight goes to a draw, then it is counted as a loss. In contrast, if you bet on a fighter outright to win, and the fight ends in a draw, then your bet is refunded. The reason for this is because when you placed your straight wager, a draw was not considered a part of the possible set of events, whereas with your proposition wager, a draw was. A lot of people lost money on Jon Fitch against BJ Penn because they had placed a proposition wager that Fitch would win by decision, and it is something to always keep in mind.
The line on Jon Jones vs Mauricio Rua is -185 Jones, and +160 Rua. As mentioned in this article (finding value in lines theory), these lines can be converted to percentages, and in this case those percentages add up to ~103.5%. Further, recall that those percentages add up to more than 100% so that the bookie makes a guaranteed profit. Imagine if instead the line was -200 Jones and +145 Rua, then the percentages would add up to ~107.5%. The important conclusion here is that the higher the sum percentages of all possible events in a set, the more juice there is on the lines.
However, when dealing with proposition bets, there is typically much more juice on the lines. If you were to add up the percentages for, "Jones wins by decision" + "Jones wins inside distance" + "Rua wins by decision" + "Rua wins by decision" + "draw", you would get a number closer to 150%. As a caveat, just because there is a lot of juice on the combined lines does not mean that it is impossible to find value on any of them.
I will spare you the details, but you can only parlay independent events. Two events are independent if the odds that one event happens remains unchanged regardless of whether the other event is known to have happened. As an example, you could never parlay "St. Pierre wins inside distance", and "St. Pierre wins by knockout", because the odds that St. Pierre wins inside the distance given he wins by knockout is 100%. (Although, this number need not be 100%, it only needs to be greater than the percentage chance that St. Pierre wins inside the distance). So, if you are trying to parlay propositions and the bookie is not letting you, this is probably the reason.