There is a subtle difference in how that word is used here. Shock is a no-no. To shock is 'okayish' The difference is that Shock is a product, whereas to shock is part of the sustained process of SLAMming. Where you raise the FC to a level that can kill algae and then maintain that level until all evidence of the green meanie is gone.
Properly maintained FC level in relation to your CYA level means you really should have no need to SLAM.
Properly maintained FC level in relation to your CYA level means you really should have no need to SLAM.