Mosiah 21:13

And they did humble themselves even to the dust, subjecting themselves to the yoke of bondage, submitting themselves to be smitten, and to be driven to and fro, and burdened, according to the desires of their enemies.

They made themselves very humble. They let themselves become slaves. They let themselves be hit, driven around, and made to carry heavy loads. Their enemies could do whatever they wanted to them.

They humbled themselves completely, accepting the yoke of slavery, submitting to beatings, to being driven around like animals, to being burdened however their enemies pleased.