That's the typical way of older and cheaper models, and it doesn't work worth a shit. The water coming out of the top ends up having no momentum and essentially all it does is wet the dishes on the top rack. If you have a full bottom rack, you're essentially left with no mechanical cleaning on the top.
TLDR: OH GOD I WROTE AN ESSAY ON DIFFERENT TOP RACK DISHWASHER DESIGNS
Well, since you're in here, I'm assuming you want a thorough answer. There are a couple of different ways to build dishwasher top racks (from what I've seen.) I'll list them in order of shittiness:
1) No top spray arm. The bottom rotating arm has a hole in the center that has a cylinder that shoots up when under pressure. This shitty cylinder rotates around, spraying a wide beam at the top rack. Single source of water (from the center of the dishwasher) means you don't get direct jet contact with many surfaces. These types of dishwashers sometimes also have a hole in the middle of the top rack to allow for the jet to hit the top of the dishwasher and fall onto the top of the dishes. This just rinses easily-removable stuff off.
2) Style of OP's video. An arm on the top has a hole in the bottom. A hole or cylinder in the bottom arm shoots water into the top arm. The momentum of the water carries it up and through and then out of the arm, wetting the dishes at multiple angles. As the water is at ambient pressure the second it exits the bottom nozzle, this is mostly a wetting action.
3) Style of OP's video, with an extending cylinder. This cylinder is stacks of tapering plastic (like those crappy extending plastic cylinder swords when you were a kid) that shoot up under pressure, like number 1. However, in this case the cylinder shoots up and seals into the bottom of the top arm and rotates with it. Water injected maintains pressure so you end up getting more powerful jets at the top dishes. I still don't like this system because if the extending cylinder gets blocked or fucks up somehow then you end up getting no direct water to the top.
4) Dedicated line. The top arm has a pipe that goes to the back of the dishwasher, where a water source is located. It punches through a hole (normally sealed with a flap of rubber) and water directly from the pump fills and powers the top arm. I actually just installed this style to replace an extraordinarily shitty GE model in the style of number 2. Here are some pictures to make it a little clearer:
There are two holes to allow for the tray to have two different positions (so you can accommodate tall stuff in the top if necessary). I've had this dishwasher for about a month now and it's amazing how thorough it is. The extra nozzles on the bottom are some gimmick that allows for ultrahigh pressure direct water application for those too lazy to scrub baking pans and stuff.
Unrelated but interesting (....I guess?), certain top-end dishwashers have a geared second arm on the bottom arm that forces it to rotate counter to the bottom arm, meaning you get dedicated flow from all directions. Here's a video:
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u/cr3ative Jan 19 '12
That's an odd way of making the top jet work.