To understand this, first you should know about different solutions and concentration.
Less concentration of water in cell. Cells shrinks.
More concentration of water in cell. Cells bulge.
Equal concentration of water in and out of cell. Cell size is same as before.
When a person drinks salt solution, the condition in his body is hypertonic and the cells shrink because salt level is higher than required amount. That is water concentration is low. The body tries to expel this excess salt content.
The best method adopted by the body is to vomit.