The way Shields absorb hits is fairly simple. If a Hilichurl hits you for 1000 DMG, and you cast a Shield that has 4000 Base Absorption, the Shield will block 4 hits from the Hilichurl. However, Shields can simultaneously break and cause you to sustain damage within a single hit. For example, if your Shield's Base Absorption was actually 3500 instead of 4000, your Shield will fully absorb 3 hits and only half of the fourth, causing you to take 500 damage.