the "bond" works as such.
Airline x offers you a job with a salary of 35k. If airline x is part of the program you can choose to take a "salary" of 23k and have the remaining 12k per year tax free. You still earn the same amount but save a few hundred on tax. Think of it as tax relief, for now. I say for now because HMRC are now looking closely at it.
To say that the course is fully sponsored and you get 70k back or however much it is now is a lie.