Looking down into the pump module cup. You can see the cup inlet flapper valve (blue/orange device). The S2 flapper and inlet hole are larger than the S1 pump module. As the pump pulls fuel out of the cup the higher fuel level outside the pump module will push open the flapper valve and flow into to equalize it. You can also see the siphon device that has it's only siphon container housed within another smaller cup in the main fuel cup.