I may have been under the wrong impression but I always thought backlash at the beginning of the cast is controlled by the spool tension or cast control and backlash at the end is controlled by brake settings. The SLX MGL 70 instructions say just the opposite. 
“if backlash occurs at the beginning adjust with Shimano Variable Brake System. If backlash occurs at the end, adjust friction cast control.” 
It’s my first Shimano. Is there something different with this brand?

Centrifugal brakes work at the beginning of the cast as that's when the spool is spinning the fastest.  When you launch your bait, those brake blocks move outward and make contact with the brake drum (or plate depending on reel) to slow the spool.  If you're adjusting spool tension, that tension is applied throughout the entire cast since the tightening of the cap applies pressure to the inner races of the spool bearings.  Adjusting spool tension is fine to a point, but it can also introduce undue stress on your spool bearings if you overdo it.  Personally, I'd work on my thumb more than I would be messing with the spool tension.  You want your tension set to where very little side play is present, and some reels you don't want any at all.  Experiment.

3 minutes ago, redmeansdistortion said:

Centrifugal brakes work at the beginning of the cast as that's when the spool is spinning the fastest.  When you launch your bait, those brake blocks move outward and make contact with the brake drum (or plate depending on reel) to slow the spool.  If you're adjusting spool tension, that tension is applied throughout the entire cast since the tightening of the cap applies pressure to the inner races of the spool bearings.  Adjusting spool tension is fine to a point, but it can also introduce undue stress on your spool bearings if you overdo it.  Personally, I'd work on my thumb more than I would be messing with the spool tension.  You want your tension set to where very little side play is present, and some reels you don't want any at all.  Experiment.

Thanks.....that helps a lot

Yep.  Brakes slow down the initial spool speed and the cast control for the the terminal end of the cast.  That is why the old standard is to hold your rod straight out with the lure on, release the spool, and the lure should drop to the floor/deck and stop without over spinning the spool