‘Shakti’ is energy and ‘pat’ is to fall or to transfer, whereas ‘Diksha’ is initiation by Guru. So the complete meaning of ‘Shaktipat Diksha’ is ‘the initiation of disciple by Guru through divine energy transfer’.
An enlightened Guru who has become one with the universe can easily evoke the divine energy or Maa Shakti upon request. And can transfer or channelize that energy to the disciple to activate the kundalini.
The energy makes kundaini to rise up and clears all the blockages coming in its path.
There can be varying degree of energy transfer based on the stage of the disciple, Guru would know that.
To receive shaktipat, the disciple is only required to have intent / sankalp to receive.
The whole shaktipat process is watched and controlled by Maa shakti, who takes care of all its aspects.