When student aid is cut, some students make up for it by working more. Their wages are taxed, so part of the saving comes back as tax revenue straight away. (Some take a student loan instead, and some spend less.)
Of every 100 euros that a change to “Student aid” saves or brings in, 10 € comes in as extra revenue on the line “Personal income tax” every year. (The other way round if the change adds spending or cuts taxes.)
E(t) = s × T(t)
Estimate: if about half of the cut is replaced by extra work and about 20% of a student's extra income is paid as income tax, about 10% comes back as tax. Both figures are estimates.
Share of the change's own effect on the balance, year by year while the change continues: −15% means 15% of a saving comes back as cost (or 15% of extra revenue is lost). With the default values.
You can switch the effect on or off and adjust its assumptions in the simulator under Cause-and-effect chains.