A fundamental tax planning strategy is to accelerate deductible expenses into the current year. This typically will defer (and in some cases permanently reduce) your taxes. But there are exceptions. One is if the additional deductions this year trigger the alternative minimum tax (AMT).
Various limits apply to most tax deductions, and one type of limit is a “floor,” which means expenses are deductible only if they exceed that floor (typically a specific percentage of your income). One example is the medical expense deduction.