Correct, "that's the theory"
If you go back to the start of bidline as we know it now there were many pilots on standby (on the stack) at home and at the airport. It was quite acceptable if you were at the "bottom of the stack" to go off and play golf (with pager).
But with time BA (in their wisdom) reduced the numbers on standby and started to use the draft facility as standby cover.
If one has the right number of crews per aircraft, together with sensible rostering (not max duty with min rest, together with min turnrounds) it could work.
Odd "drafts on the day" were cheaper than many pilots "on the stack". However BA would ignore the "draft from volunteers from the top", then, if no takers "force-draft" from the bottom of the seniority. They had their own lists of "helpful" pilots that lived locally and were able to be "drafted on the day" at very short notice. Human nature I suppose, but not within the "rules".
However, as the workload increased the number of "volunteer drafts" (starting at the top) decreased such that rosters having had no "volunteers" following numerous telephone calls had to resort to "force drafting" (from the bottom). In theory the most junior pilot would be contacted offering a "draft", and if he refuses the offer is then hit with a "force draft". If you were near the bottom, and had refused an "offer" then keep away from the phone the next time it rings 'cos it may well be a force draft!
There were stories of pilots installing an extra phone (BA only required one number) for work calls. One chap even had a red phone! If it rang and you wanted to talk to BA then answer it, if not don't!
Although amusing the tales of wives answering phones and suggested hubby was "off fishing" were numerous. Rosters even resorted, once the wife answered, to suggesting "BA here, just to let you know you husband has got stuck in JFK" to which she (off he guard) replied "but he is next to me here!!". "Fine, in that case we wish to force draft him". Now we see modern equivalents in the form of "caller id" and "call barring" taking the place of the well trained wife!
Pilots are a reasonable bunch, and will "go the extra mile" for the company as long as they don't feel the company is taking the p*ss. At the moment they feel they are.
Just a last thought, scheduling is always difficult under bidline at the beginning of each month with trips carrying in from the previous month clashing with the first trip(s) of the new month. When half term/ bank holiday weekends are also at the beginning of the month it can be desperate. June is one such month. BALPA approached the company months ago pointing out the problem and suggesting they reduce the number of trips overlapping into June, together with reducing SEPT, Simulator, route checks etc for the first few days of the month. The suggestions were ignored. BA are now paying the price with cancelled trips.