This law comes from several experimental observations like for example the fact that some molecules take all the same shape although in theory several shapes are possible.
Sheldrake explains that by the existence of a particular physical field which he calls morphic field, and which would resemble to matter fields (wave functions of particles).
When some physical entity placed in some conditions has some behaviour, according to Sheldrake this would modify its morphic field which would propagate so that later when a similar entity will be in similar circumstances, it will enter in morphic resonance with this morphic field and its behaviour will be influented by it. A picture of this phenomena consist in imaging a soft surface with balls rolling on it. These balls dig canals behind them. When a ball is near a place where another ball has already gone, it will tend to go down to the canal and its trajectory will be deviated so that it will go near the first ball.
This theory could explain the formation of the laws of physics in the following way : at the origin, these laws were not determinated, then the particles behaved in some way, creating so some configuration of the morphic field, and then the particles tended to behave in a similary way, creating the laws of physics. We mignt also connect the notion of morphic field and formative causality with the problem of the wave function collapse. We have a phenomene, the collapse, for which we know no law, and a law, formative causality, which applies to a morphic fields whose exact nature is unknown. So we can suppose that the law of formative causality could apply to the collapse of the wave function, and could be expressed more precisely so : if some wave function collapses in some way a similar wave function will have a higher probability to collapse in a similar way.
This mechanism is very general, and we can often find it in nature and society. We can compare it with :