How does the mob grinder work?

There is a tutorial describing how to build a mob farm. It uses following pattern:

S = Stone (or other solid opaque block)
W = water 2 levels under the stone
H = hole
B = border wall

|--------------20 blocks--------------|
B B B B B B B B B B B B B B B B B B B B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B W W W W W W W W H H W W W W W W W W B
B W W W W W W W W H H W W W W W W W W B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B B B B B B B B B B B B B B B B B B B B

When the farm was built, mobs started to fall out immediatelly. This is a little suprising. If they appear on the stone platforms, shouldn't they just wander and only randomly fall in the water streams? How come they fall in so frequently? Do they walk there on purpose?

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How does the draconic mob grinder work?

Mob Grinder is a block added by the Draconic Evolution mod. It can be used to kill mobs in a 9x9x9 area in front of it. The front of the machine is marked with a skull and crossbones.

How do you use a mob grinder in Minecraft?

Building the mob grinder

Dig a 2x2 hole in the ground. It only needs to be one layer deep. ...

Place your four treasure chests into the hole, all facing the same direction. ...

Place your four hoppers on top of the treasure chests. ...

Place your slabs on top of the hoppers.

How far away do you have to be for mobs to spawn in a mob grinder?

Mob spawning in Minecraft is limited to a 128 blocks distance from a player. Experienced Minecraft players create large, efficient farms in a sphere of 128 blocks radius.

If Your Mob Grinder Isn't Working, Watch This!

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Images: RT Poole, Anamul Rezwan, Pixabay, Pew Nguyen