One hand stays on your dog: the shampoo sits inside the handle, so you never let go to reach for the bottle.
No leaning into the tub: the long handle gets to a back, a belly or a back leg without you climbing in after it.
Soap goes where the dirt is: it comes out of a hole in the middle of the head, so it starts at the skin instead of sitting on top of the coat.
It dries where you hang it: there is a slot in the end of the handle, so it goes straight on a hook and drips into the bath.
Your dog gets a scratch, not a scrub: the spikes are soft rubber, so the soaping part feels like being rubbed rather than washed.
Where the shampoo actually ends up
Poured from a bottle, most of it lands on the outside of the coat and rinses away before it does much. On a thick coat you can use half the bottle and still have a dry dog underneath.
Here the shampoo comes up through the centre of the brush head, under the spikes. It starts at skin level, right where you are already working.
The part of the bath your dog does not mind
Most of a bath is something that happens to your dog. The water, the noise, the standing still.
The soaping is the one bit that can go the other way. Soft rubber spikes moving in circles are close enough to a scratch that plenty of dogs stop bracing and lean into it.
What comes away in the water
Watch the spikes as you work along a back. Loose hair lifts out of the coat and collects between them, and you can see it come away when you rinse the head under the tap.
That is hair that would otherwise have come off on the sofa over the following week.
Questions
Will it get through a thick coat?
Yes. The spikes part the coat rather than skating over it, and the shampoo is already coming out underneath them.
On a dense coat like a Golden Retriever or a Husky, work in small circles and let the head sit in one spot for a second before moving on.
What can I put in the handle?
Any dog shampoo you already use. Thicker ones flow better with a little water added, so mix it in the handle and give it a shake before you start.
How do I clean it afterwards?
Rinse the head under the tap and the hair comes straight out from between the spikes. Run clean water through the handle if you are not using it again for a while.
Then hang it by the slot in the tail and it dries on its own.
Is it soft enough for a puppy?
It is. The spikes are moulded rubber with rounded tips, not cut bristles, so there is nothing sharp to catch on thin skin.
It is a reasonable way to get a young dog used to being handled in the bath, since most of what they feel is the rubbing.
