how does it work?

Do you need to understand the Math behind SAIF rate to use it?

Not usually, the measure is usable as a concept just by understanding that SAIF is a mathematical representation of injuries with two-weeks lost time.

Here's the math anyway.

The math behind the SAIF rate involves adjusting the numerator of the injury rate.

The benchmark injury (2-week LTI) has a value of 1 for both the Injury Frequency and the Severity Adjusted injury frequency. That is why it is the mathematical benchmark. Every other category has a base score that is set by a base-3 logarithm (each category is worth 3x the previous). Each category also has a modifier for more or less severe injuries in that category

SIF: Severe Injury or Fatality - Base Score 3 - Modifier - Fatalities are doubled (6)

LTI Lost Time Incident - Base Score 1 - Modifier - Under 14 days (0.11 +0.89(days/14))

RW Restricted Work Incident - Base Score 0.33 - Modifier - Under 14 days (0.11 +0.22(days/14))

MT Medical Treatment - Base Score 0.11 - Modifier - Limited Medical Treatment is half (0.05)

Note: The modifier on LTI and RW is (medical treatment score + Remaining category score*% of 14 days)
Limited medical treatment is when someone gets a scan or prescription with no other medical treatment.

Both Injury Rates, and SAIF Rates are typically presented as 12-month rolling numbers.