RF (Radio frequency radiation) does not work that way. Most likely, you are just raising the transmitter to a high enough level to pass over more obstructions. You would get the same effect by simply holding the key thingy over your head, and then not look like such a twit.
Let's break this down...
Your mouth and skull somehow focuses the RF energy forward or spread the signal outward in all directions. In the first case, this would only work if you already knew the direction of the car (thus focusing the energy in that direction). In the second case, unless you have an RF amp in your head, dispersing the signal over a wider range will decrease the strength of the signal.
But, this is not how RF works. Look at a radar dish; radar works on RF, and the dishes are made of a material that will reflect RF. The dish is shaped as a parabola so that the reflected RF is focused in a certain direction. Now, if we look at a radio or cellular antenna, it is just a rod made of metal. The idea here is to disperse the signal 360 degrees. Though these two types of antenna have a different desired outcome, one thing they have in common is that they are not made from flesh and bone. Why? Because flesh and bone are horrible for reflecting or transmitting RF radiation.
RF, at low levels, passes through the human body. Just as it can pass through concrete walls, plaster, electrical wiring (if it isn't too dense), plastics, glass, etc. What it can't pass through are engine blocks, metal rims, rebar, anything metal. In other words, the things you might find in a parking garage. Most of which are found below shoulder height. If the guy in the video had merely raised the transmitter to the same height and NOT pointed it to his chin, he most likely would have had the same result.
Now, for those who brought up the rabbit ears and such... the receiving end is a slightly different story. RF creates an electro-magnetic field and by touching the antenna, moving them or even moving yourself, you are altering the field around that antenna (yes, even you have an electro magnetic field). That's because the signal is weaker at the receiving end, and small changes make a big difference. I do not recommend trying these at the transmitter, because you will be badly burned, and the signal will go out as normal.
