CPF SA povides 4% interest and can be used for investment and retirement. Yes, you can move OA monies to SA to bump up 2.5% return to 4% return. At age 55, the Retirement Account (RA) is created. And you'll be asked to set aside the Full Retirement Sum (176k in 2019, 181k in 2020). Money from SA will first be pulled into RA, and if insufficient, OA money will be next. CPF SA shielding is just to make sure more OA money is used to create RA instead of SA money. Example: You have 200k in OA and 200k in SA and you're turning 55 today. Typically, after RA is created and FRS is transferred, you'll be left with 200k OA, 24k SA, and 176k RA. With shielding (investing 160k in SA and then transferring back after RA is created) it'll be 64k OA, 160k SA, and 176k RA. As you can see more money is earning a higher return as compared to without shielding.