The 'Mobile H Perception' Option is located in the 'Options' panel in the upper third of the InChI Generator window (see image below). In this example the structure that has been entered is pentane which has no mobile hydrogens. The program recognises that the entered structure has no mobile-H and greys out 'Mobile H Perception', letting you know that even if you could toggle the option, it would have no effect on the InChI generated.
So now we will look at an example structure which does contain mobile-H, ethanoic acid. The structure we will input is as below.
In the image below, the structure has been entered into the generation program and we can see that the 'Mobile H Perception' option is no longer greyed out, showing that the program has recognised that the structure contains mobile-H. On entering the structure, the InChI generation program has normalised the mobile-H and indicated that it is shared between the two carboxyl oxygen atoms. It is worth remembering here that one of the main aims of the InChI project is to be able to fully normalise input structures, so that InChIs that are generated do not depend on input drawing conventions.
Note the dashed double bonds and that the two oxygen atoms have been labelled with the same 'Non-stereo class' and 'Mobile group id' indicating their equivalence.
More importantly, however, is the fact that this information is shown in the InChI string. We see that this InChI contains the three sub-layers of the Main layer (chemical formula, atom connection and hydrogen).
In the hydrogen sub-layer we are informed that the atom with canonical number 1 has three H atoms bonded to it (indicated by 1H3) and that one H atom is shared between atoms with canonical numbers of 3 and 4 (indicated by (H,3,4)), the two O atoms.
So what happends if we now uncheck the 'Mobile H Perception' box? By doing this, we are telling the InChI generation program that we want it to represent the structure with all H atoms fixed to the positions they held in the input. Thus, this method could be used if you wished to represent one particular tautomer of a molecule.
As you can see above, immediately after unchecking the box the program redisplays the structure without mobile-H normalisation (you will notice the solid bonds to oxygen, and that the 'Non-stereo class' and 'Mobile Group id' no longer appear in the legend) and also displays the InChI for this 'Fixed-H' structure.
The new InChI that has been generated is basically the previous InChI with a new Fixed-H layer appended. This new layer represents the fixed positions of those hydrogens that were considered mobile. Hence for this structure the Fixed-H layer only contains information for the H that was considered mobile between the two O atoms.
In fact, this example highlights one of InChIs great strengths. As different types of structural information in an InChI are held within different layers, then on refinement of a structure the initial set of layers are not affected and the only change to the InChI is that a new layer is appended. This allows you to deduce structural similarity by looking at InChIs alone, as explained in the FAQ here.