The idea is; you pump water to the bottom of your bog. When/if you ever have to clean it, you'll drop an additional pump down into your cleanout snorkel system. To do this right, so I was told by a pond builder, is to force water from a high volume pump (your main pond pump for instance) DOWN through the top of your bog. This forces the clogged particles to drop into the cavity area below where it SHOULD flow to your lowest point (the bottom of your cleanout vault). The other pump then discharges until you see the water run clear.
As you build this (and it's the mistake I made because the directions weren't as clear as they should have been) is to taper your bog walls to a point, that being the line where you sink your main centipede/bottom bog tubing. This is the tubing with slits where the pond pump forces water. This line of pipe also pitches to the low point (the vault cleanout). If you do this, you'll get a flow (upon surface force water) toward your other cleanout pump in the vault. I was told it takes a two three times to fully clean a bog.
What I used, if you're interested; I used 1-1/2" flex pvc from the pond pump joined to 4" corrugated (in which slits, every 2" were cut, facing down) drain pipe. I angled this along the bottom in it's own 6" trench toward my makeshift cleanout vault. You need this vault/vertical tube to be large enough to house your cleanout pump (if and when you use it). This vertical tube to the vault is capped off when not in use. I didn't use Aquablox (didn't know about them and now that I do, seriously too expensive imo) but instead, used large, 8" river rock as a base layer and then one more over top the drain pipe run. This gives support so the tube won't crush. On top of this, I laid 12" of 2" river rock/cobble and finally, 12" or so of 3/8 pea gravel. Plants then grow over top.
NOW, HOPEFULLY, you'll never have to do this and the minute particles that enter your bog will eventually decay naturally and without your intervention. If you force water too fast or too much through your bog, you'll be more prone to clogging. Also, slower gives your plants more time to take up nitrates and your nitrifying bacteria time to do their thing. And very important, don't have your pond pump on the bottom of your pond--lift it off at least 12".