When you pay your monthly contribution, it splits into two streams. The 'risk' portion goes into the shared scheme pool to fund hospital claims, chronic medication, PMBs and large medical events. The 'savings' portion (on Saver and Comprehensive plans) goes into your own MSA for day-to-day claims. Hospital-only plans are 100% risk; there is no MSA on them.
SA example: On Discovery Classic Saver, of your R3,880 monthly contribution, R3,104 is risk (80%) and R776 goes into your MSA (20%).