I know you can recite them and it has an effect, i have witnessed it myself, so i am now studying it further. In kabballah it is taught that the cause of all material changes are higher dimensional forces in movement, which is the theory of magick. We use the mental plane to affect the spiritual plane, which then affects the material, as taught in hermeticism, the three planes of existence. All things material have an expression and cause in the higher dimensions, so words and symbols can be impressed on by the mental which causes and affects the spiritual realm and then further down in the material again.
In other words, the mantras have a spiritual charge that you created in the higher realms, which affects the physical reality. I have seen both anecdotal proof and written accounts of occultists charging a mantra or a symbol, show it to others to affect them in some way, which usually has more subtle or covert changes to someone. It appears that the reason mantras and symbols work best if you know them is because you are purposefully inviting those energies in and creating them yourself, but hidden symbols and still have an effect.