Currently, the "stay on trails" option for creating courses works fine with Google Maps when you are looking at trails and footpaths (through forests, etc.) However, this does not work with OpenStreetMap data. Trying to route on a marked trail or footpath using OSM data with "stay on roads" checked forces you back to the nearest car road. Given the behavior with Google Maps source data and this checkbox, I'd say it's not working as intended. That's a shame, because OSM has better trail information in many locations and it can be very quickly updated by interested users who want to plot courses and share the trail info with others -- unlike Google Maps.
A better possible solution given the number of outdoors enthusiasts who would prefer to stay off car roads, but on footpaths, might be to include a checkbox option that is something like "stay on trails," where it would follow trails, routing onto car roads only where trails disappear between points A and B.