The science of vashikaran (hypnosis)

There's an asana called savasana or yoga nidra which anyone can do. A trainer, by using verbal cues, repetition, imagery etc heightens the concentration and focus of participants. This relaxes them and makes them amenable to suggestions  from the trainer.  This is how hynposis also works.

An alternative to this technique is used in rural india today. In a dark room lit with few lamps,  with pictures in brilliant red (long wavelength) colors and with certain sounds, priests shift individuals into a high concentration and focus state and make the person before them to do what they want. 

Many swamijis/gurujis help their disciples arrive at the same concentration and focus (leading to mind relaxation) by combination of their voice, music, room settings, chants, decor etc etc..

The above techniques were used for long times as a cure to psychotic disorders by relaxing the patients (like how hypnosis is used today as an augmented clinical therapy). In many cases of depression, the concentration and focus helps relax the mind (with no distractions) and helps people cope with depression or stress. 

All the above are just another forms of hypnosis for mind relaxation. But their effects wane over a period. Hence they are good band-aids and not cures. In many cases, they can help people cure themselves.

In short, vashikaran or hypnosis uses different ways to improve concentration/focus leading to mind relaxation leading to a state in which the person becomes amenable to suggestion. The mind relaxation could be used to augment other cures or remedy stress/depression to some extent.

This is the success factor behind several priests in rural India and swamijis in urban India.

There's another type of vashikaran also, which is black magic. 

In this a person A tries to do vashikaran or blackmagic on party B through a specialist using B's body parts or used items. Only thing is, this vashikaran is actually not of B, but that of A himself. The specialist cleverly deploys one of multiple of the hypnosis tricks (as I wrote above) on party A and makes believe that party B vashikaran is successful.  In reality nothing happens to party B, but party A believes that B has got affected.