With the support of two players assisting the jumper in the air the ball is won by the call misleading the opposition. If the opposition does contest the throw they will only be successful if they guess the throw correctly. They guess incorrectly, the opposition is prevented from recovering because the mini-unit of three players, the jumper and the two supports occupy the greater share of the space between the two lineouts.
This is the reason why defensive lineouts infrequently contest the ball only doing so after studying their opposition closely and then only contesting when they have a good chance of success. Variables the effect this are field position, lineout numbers and the tactical use the opponents are likely to make of possession. If they are uncertain they contest the space by driving the attacking team back once the ball is caught.