As an example, if you were to add 3 gallons of 6% bleach to a 15,000 gallon pool that has 30 ppm CYA, then the following shows the change in various water chemistry parameters after adding the bleach and then after it is used up:
FC ..... 0 ... 12.3 ..... 0
pH ... 7.5 .... 8.1 ... 7.5
TA ... 100 ... 109 ... 100
DC ..... 0 .... 0.24 ..... 0
If you first lower the pH with 24 fluid ounces of Muriatic Acid, you get the following sequence
FC ..... 0 ..... 0 ... 12.3 ..... 0
pH ... 7.5 ... 7.2 ... 7.6 ... 7.2
TA ... 100 ... 94 ... 103 ... 94
DC ..... 0 ..... 0 ... 0.30 ..... 0
The "DC" is disinfecting chlorine (i.e. hypochlorous acid) so you can see that first lowering the pH does result in more disinfecting chlorine, but not by as much as one might think with the traditional chlorine graphs. The reason is that CYA "buffers" chlorine (specifically, hypochlorous acid) so resists changes in its concentration with changes in pH. So normally when shocking a pool at reasonable CYA levels one doesn't worry about the pH jump, but if shocking at high levels for mustard/yellow algae or at high CYA levels, then lowering the pH first makes more sense. Normal shock level is a disinfecting chlorine (DC) of around 0.30
Now if you needed to lower your TA level anyway, then adding acid before shocking does two things at once as you'll end up with lower TA as you could see above (and, if desired, you could then let aeration make the pH rise without any change in TA).
Repeating the same analysis at a higher CYA level of 80 ppm and using 8 gallons of 6% bleach I get the following:
FC ..... 0 ... 32.9 ..... 0
pH ... 7.5 .... 8.8 ... 7.5
TA ... 100 ... 123 ... 100
DC ..... 0 .... 0.21 ..... 0
If you first lower the pH with 30 fluid ounces of Muriatic Acid (it takes more since the CYA buffer is slightly stronger and I still assumed the same 100 for TA), you get the following sequence
FC ..... 0 ..... 0 ... 32.9 ..... 0
pH ... 7.5 ... 7.2 ... 8.5 ... 7.2
TA ... 100 ... 92 ... 115 ... 92
DC ..... 0 ..... 0 ... 0.25 ..... 0
If you add a larger amount of acid, 60 ounces, to get the pH down to 7.0, then you have the following
FC ..... 0 ..... 0 ... 32.9 ..... 0
pH ... 7.5 ... 7.0 ... 7.8 ... 7.0
TA ... 100 ... 84 ... 108 ... 84
DC ..... 0 ..... 0 ... 0.30 ..... 0
So significantly lowering the pH before shocking makes sense when you have to add a lot of chlorine to get to a high FC, but it's not as necessary at lower normal shock levels.
Richard