Double Down After Split

[TMNLanc]

In Norm's book (Modern Blackjack) within Chapter Minus 1, Section "Variations/Double Down Rules/Double After Split (DAS)" the last sentence says "You must change your strategy to realize the full gain in advantage". Could someone please expand upon this. I play these Double Downs like I would any other Double Down (e.g. dd on a 9 vs a dealer 3 thru 6) but Norm's statement suggest there is more that I need to know. Thanks in advance !!!!

[RS]

You would split less often if there is no DAS. This is because some splits are defensive splits (lose less) rather than offensive (win more). 8,8v10 is a defensive split. But if you can't DAS when you draw a 3, you're not shortening the gap between -EV and +EV that much. (Btw, idk if you would or wouldn't split 88v10 in an nDAS game, just using that as an example.)