I understand the principle behind these clutch systems, and it seems to make a lot of sense. I notice some systems (SRAM) have the clutch engaged permanently, while others (Shimano) it's a toggle.
What I don't understand is the apparent ability to engage or dis-engage the clutch. When would I do this? Before a downhill? Before an uphill? Cross country? When my bones tell me chain slap is imminent?