Can you explain this? I didn't get it...
I can try
Let's say that you have an income of 200k and that if you don't make any contributions you have to pay 50k in taxes and you're left with a 150k.
Now if you take a 100k out of your ETF to put in your 2nd pillar, your amount of income tax will be lowered. You might only have 18k to pay now and would be left with a 182k. Why not, but it's better to re-invest the tax savings if you want to keep things equal (150k left to enjoy life) and maximise your returns, so instead you put a 143k in your pension (43k from your income) and now you only have to pay 7k in taxes and are left with a 150k.
So, part of your retirement fund just went from a 100k to 143k, thanks to tax savings. Of course, the returns will be lower now and that's the reason you should only do it when you're close to retiring or the ETF you were using might outperform the returns you get from the pension fund and you would have done all this for nothing.
And there is usually a catch and taxes have to be paid at some point. In this case, it's when you withdraw the amount from your pension fund, but the tax rate is much lower, so in effect, you can manage to increase your retirement fund significantly by moving things around, if you have the right pension plan at the time.
Disclaimer: Those are just my observations. I invite you to verify with your own data and calculators

. There are also lots of variations possible if one wanted to optimise based on market conditions, etc.