Puppet Master is a Prowess, this special kind of Feat can't be acquired until the third level. A Prowess is a special action that can only be done once per scenario. Unlike normal Feats, it can be acquired several times.
You hypnotise someone for hours and implant a program deep into his mind.
The program is triggered each time a very specific condition is met, for example :
"Each time you hear a certain word"
"When you feel you've been discovered by the inquisition"
"Once, one year from now"
The program is any action you want the person to make. It cannot be something the person is very unwilling to do.
For example, you can't force someone to disclose something he doesn't want to disclose or you can't force someone to suicide. But you can force someone to drink a "healing potion" which is in fact a very potent poison.
Examples of programs and triggers :
"If you are tortured by the inquisition, loose all ability to speak / scream incoherently"
"Each time you see the leader of this criminal organisation, attack relentlessly everyone around you"
"One year from now, drink this potion"
Depending on the willpower of the person, he may resists the hypnosis (with the Feat Courage in particular) or he may need several sessions.
The program stays implanted in the mind of the person for a certain amount of time, it could be weeks, months, years or a lifetime, depending on a lot of factors.