Lay-to-back means laying a horse and backing it back at a longer price after it drifts. The profit maths mirrors back-to-lay, but the risk while the trade is open is the full liability.
Maths
Back stake = lay stake × lay odds ÷ back odds
Profit on every outcome = lay stake − back stake
Profit on every outcome = lay stake − back stake
Lay £20 at 4.0 (liability £60) and back at 4.3: back stake £18.60, profit £1.40 before commission.
Risks
A horse you laid can shorten sharply on a late gamble. Set a maximum liability and a closing point before you enter, and never leave a lay open into a race you didn't intend to bet on.