A tricky subject!
I have worked out several bonds and they have varied significantly in style.
The first one was quite clever: A three-year bank loan in my name was arranged by the airline and paid to the airline to cover the type-rating training costs, with the airline making the monthly repayments, stating from the date of the loan, while I worked for the airline. If I left the airline, the repayments from then on were my responsibility (they did have some protection, maybe their own insurance, that if the airline went bust the loan would be paid off so no more liability for me).
The second one was more thought-out: The bond agreement itemised the costs of all of the training with the date of commencement of the bond being the date that I got the type on my licence; the situation that Barcli mentioned could not happen there as their lawyers had already thought that out. (I needed to have a witness to my signature on the agreement and my non-flying friend who did so found the concept and figure to be eye-watering!).
In both airlines any new type-rating bond would cancel any existing bonds.
Subsequent bonds have been far less thought out and I am convinced that could easily have led to the 'get out' that Barcli mentioned. There are also some rather 'woolly' terms, such as when the bond starts, that could also be contested (neither of my first two bonds left that sort of 'fuzzy' loophole).
But this is a small industry. Think very carefully about the situation that thetimesreader84 mentions. (I have been employed by the same Flight Ops Director in two different airlines about 4 years apart, so you don't want to muddy waters behind you!) But that goes two ways too: I have had an airline manager say to me "We don't know the management at xyz, but we know about the management at xyz"! I knew two pilots who 'jumped' very harsh bonds (huge amounts for a very long time) abroad, they both ended up in jobs in major UK airlines and at their interviews each has 'come clean'and said that they had jumped this bond and in both cases had the answer "yes, we know"!! So it does work both ways.