The Low-ball works by first gaining commitment to the idea or item at lower costs which you are confident that the other person will accept, then using the fact that people will behave consistently with their beliefs to sustain the commitment when you change the agreement at a higher level. This strategy is an illusion of irrevocability whereby a person believes that a decision made (at lower costs) cannot be reversed, he/she may consider a handshake instead of handing over the money as the final transaction to close that deal hence they have a responsibility to commit to it to the very end. Besides, agreeing to a low price creates excitement and not buying after this state is induced may lead to an equally deep depression, which the person may avoid by continuing with the more expensive but (apparently) reasonable purchase. When the final price is not that much higher than elsewhere, the person weighs up the inconvenience of going elsewhere with the short-term benefit of holding their purchase.
The trick of a successful low-ball is in the balance of making the initial request attractive enough to gain agreement, whilst not making the second request so outrageous that the other person refuses.
“The mindlessness of ostensibly thoughtful action: The role of "placebic" information in interpersonal interaction”. http://eva-elba.unibas.ch/index.cfm?w=53&f=527&c=3200&file=/Langer_et_al._(1978).pdf