Hello Del,
As you've already guessed, trunk pre-select allows you to force a call out over a particular trunk. Simply prefix your dialled number with the pre-select value. So, if you've given a particular trunk a preselect code of 91 then by dialling 91+number then SAIL will use that trunk, irrespective of what you have defined in your routes.
Pre-select was primarily put into the system as a diagnostic aid for analogue & ISDN lines but you can use it however you wish.
Best
Selintra