Think about what a high count means. It basically means the next cards drawn are more likely to be tens.
That helps the chances of your doubles. It makes splitting more attractive. It hurts your chances if you hit a stiff hand. And it also increases the chance the dealer will bust.
Here’s what a deviation is telling you to do:
Hard hands 11 or less: double instead of hit when the count is high enough
Hard hands 12 or more: stand instead of hit when the count is high enough or hit instead of stand when the count is low enough
Soft hands: double instead of hit/stand when the count is high enough
Pairs: split when the count is high enough