There is no any magic in the backtesting algorithm of Forex Strategy builder.
I'm working on it every day during the last 8 years and finally it is more reliable and 1000 times faster than the MetaTraders's one (best regards to it's developers ).
Actually the algorithm is extremely simple and this fact make it extremely fast. On my laptop the program makes 50 backtests per seconds that means it can generate or optimize a trading strategy for every market tick.
I cannot announce the algorithm now, but probably I'll make the program open source some day. Then I'll explain it to everyone.
If you are in doubt about its correctness use the intrabar scanner. The scanner uses all available shorter periods. The scanning will not show any difference in the backtest's result when there are no ambiguous bars. So the algorithm does no need intrabar data for most of the strategies. When the strategy backtest shows ambiguous bars the scanning improves the reliability. Some times even the shorter periods cannot be enough to precise the price route inside some bars and in these cases FSB uses different interpolation methods (pessimistic, optimistic ...).
That's all for now. If you have more questions I'm here.