Back-to-lay means backing a horse and aiming to lay it later at a shorter price. It's the most intuitive trade because the risk while open is limited to your back stake.
Maths
Lay stake = back stake × back odds ÷ lay odds
Profit on every outcome = lay stake − back stake
Profit on every outcome = lay stake − back stake
Back £50 at 3.0 and lay at 2.94 (3 ticks) for £51.02: a £1.02 green before commission.
Risks
If the price drifts, you either close for a loss or hold the bet into the race. Holding turns a trade into an ordinary bet, which is fine if that's a decision you made in advance and not a hope.
Tools for this
Calculator
Back-to-Lay Calculator
How many ticks a price must shorten to lock in the profit you want.
CalculatorGreen-Up Calculator
Hedge an open back or lay so you win the same on every outcome.
CalculatorTick Calculator & Ladder
Move a price by any number of ticks, or count the ticks between two prices.