[Answer courtesy of Emily Grant.]
State reimbursement program for special education costs incurred the a school district
Reimbursements from the state are for the previous year expenses
Any costs that are above 4 times the state average per pupil is eligible
State pays 75% of cost over the threshold
Example: In FY19 the state average per pupil cost was around $16,000. Four times that is $64,000. If an out of district tuition is $120,000, then the state will cover 75% x (120,000-64,000) = $42,000. The total of that for all students placed out of district in Natick is the Circuit breaker revenue. The circuit breaker expenses would be the amount paid out for tuitions.
The school maintains a revolving account (Circuit Breaker Revolving Account) where funds are deposited and pulled out of.