Laing horse in play
You need to have a good read of the manual. https://www.betangel.com/user-guide/
An offset is a certain amount of ticks (or percentage of price) from your initial bet. The best market price at the time of your manual lay bet will be less than the price you laid at, and would guarantee a loss.
If you lay a horse with a one-tick offset, if a better price is available, your bet will automatically be taken at the better price. That's the Betfair price-matching algorithm. No action need on your part.
Have you actually tried it?