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RE: "how would this be any good at finding a knife in my pocket? does it penetrate ??"
No. Read my very brief explanation to Ernest in post #15.
There is no magic. I'll add. Think of it this way. A very short pulse of laser energy is focused on let us say your wrist. The laser beam is extremely very short in duration so it would not destroy hardly any tissue on your skin surface.
But would ionize various types of molecules located at your skin surface in that tiny area of space. The energy that would be produced at very precise frequencies would then be sensed by a "sniffer unit" place close to the area the beam was directed at.
The sniffer is a one or more types of what we call spectrometers. They within the bounds of the type they are can detect certain types of molecules within a range, as well as the type of radiation emissions given off in the form of say infrared, through visible through ultraviolet radian bands.
The detector will then analyze the sample it takes into it's chamber. And then display perhaps on a little tape ticker or now adays, on a little hand held computer display the type of materials found on that person's wrist. So if you where dealing lets say in chemicals relating to drugs, or explosives, just two examples, it would give you a readout as to what type of compounds where found.
Hope that helps a bit in envisioning what this type of device could afford police and security agencies.