This one came as a bit of a shock to me personally. I had used the flashlight trick for years in my paranormal investigation techniques! However, you can’t argue with the evidence – it just isn’t a conclusive means of communicating with spirits. The idea is that the investigator unscrews the flashlight until it is only just barely making contact with the battery. The slightest touch can make the flashlight turn on or off. The flashlight is then set down across the room from the investigators, who then ask a series of questions and give instructions on how the entity can answer (1 flash for yes, 2 for no, etc).
The trouble comes when we look at a few factors, namely the physics of how a flashlight works, and the subjectivity of the experiment.
This video goes into great detail on how the flashlights were experimented with, observed, and the outcomes. In short, the heat generated by mechanisms of the flashlight cause the contact points to expand and create contact. This was proven when the flashlight’s contact points were observed actively expanding after being turned on for a certain amount of time (20 seconds) and then again when the flashlight is set up for the experiment without being turned on and there is no incident of flashing on or off. The flashlight is in such a delicate state that any movement or vibration, potentially even the vibration from the voices of those who are in the room and asking the questions, could trigger the flashlight to turn on or off. The video explains it better than I can, but suffice it to say that it is for this reason that we can no longer count it as a means of communication with the spirit world.