When a level is first started there will be a lot of ojbects to take into account. Use Drake's journal. When you activate a switch or shoot a disc, symbols will appear on each mechanism involved. This allows you to track which switch does what action. During your first run through a level, activate everything and note the symbols. Discs work similarly to pressed switches, except they can be shot from a distance. Shooting a disc is not considered a turn. A disc must be in Drake's line of sight in order to be used.