The liquidity pool will offer you a conversion rate, in our case, 300 BUSD (y) for 1 MATIC (x). When you supply the 300 BUSD to the pool and remove 1 MATIC, it will have a higher supply of BUSD and a smaller supply of MATIC. This action causes the price of MATIC to rise as k is constant. In other words, you are using your BUSD to buy MATIC. As more MATIC leaves the pool, its price in comparison to BUSD rises.