Linesplit is one of many tricks in Agar.io. It consists of making a perfectly lined quadruple split, and can be done by putting your mouse in the exact center of your cell, then splitting. Most linesplits are horizontal, but if you make your cell move slightly in a vertical direction, you'll be able to do a vertical linesplit. However, hitting a wall will destroy vertical linesplits.
Using the Agartool script (extension) makes a horizontal linesplit very easy: Just press the “Stop Movement” button and then split four times. The position of your mouse pointer is not relevant.
A horizontal linesplit in progress