Suppose you think that Tom Allen is going to make it with Maine voters (or would like to see that happen as much as Joe).
Turns out that the markets for Tom Allen say he's still an odds-off bet. That's an opportunity. [See a full list of GOP senators and their figures, here.]
If you have $100 to donate, take $30, say, and put it into Tom's contract and put $70 into Act-Blue.
If Tom loses, you kept to your limits for political action money. If Tom wins, you get paid about 3:1, getting back $100, money that you save and "recycle" at the next election cycle.
This type of strategy works mostly with candidates who are 'considerably undervalued' - it's not so smart when the race is marked at or near 50-50, for instance.
Have a nice day.