They're added in the early stages -before you add your yeast to kill off any wild yeasts &c. they may give an 'off' taste. Then you wait 24 hours as they work by releasing SO2 -a gas, most of which will disappear in that time -then you add your yeast. If you use boiling water on your fruit then you don't really need them at this stage and as you've found out you don't always get an off taste anyway.
They're just Sodium Metabisulphite in a measured dose -so you can use them for sterilising &c. they have a side effect of preventing oxidisation of the wine (which gives sherry like tastes) and are used (generally with Potassium Sorbate) to stabilise a wine after fermentation.
Personally I don't always use them, and if I do only at the very beginning to sterilise fruit. Some people say they should be used at every racking.
Jeremy