You only mine blocks on top of your previous blocks, ignoring blocks produced by the 49%. Since you have 51%, your chain is the longest over time, so you have 100% of the mining rewards.

You can't do that with 25% (or even 40%) hashrate.

If you control 51% of the hashing power, that means you can solve more blocks than the entire rest of the network combined. Even if other nodes on the network solve a couple blocks before you, statistically, you will eventually create a longer chain of blocks and the network will switch to your chain.

But your chain has every block solved by you, giving you all the block rewards.

That's the magic of the 51% attack. You gain control of the blocks. Because that extra 1% isn't a HUGE margin, it may take a while for your chain to become the winning chain, but theoretically, it will happen.