Trailing buy is a reversed version of Trailing Take Profit for coins that are falling in value.
It's used to buy coins at the best possible rate.

To activate the "Trailing Buy" option, click the slider to turn it on.

The percentage should be set to the amount you'd like it to trail up before purchasing to avoid purchasing the tokens while they continue to fall.

Imagine that the price of ETH is going down.
John has $500 and wants to buy ETH at the best possible price.
The current ETH rate is $430, so John places a Trailing Buy order at $400 and sets the price deviation to 10%.

John's Trailing Buy actions are described in the following table.

John has purchased ETH for $385 instead of the initial $400 which is 4% lower due to Trailing Buy.

For better understanding, review the following animated demonstration.
You'll notice how Trailing Buy follows the market's price down until it rises to the required percentage set by you.