Tidal Force does not trigger the global cooldown, making it useful in burst healing macros. The bonus to critical effect chance stacks with your normal crit chance. If a player already has a 40% chance to crit from other sources, the first spell cast with Tidal Force is guaranteed to crit.
When cast, Tidal Force places a buff stacked three times on the player. Each stack increases the critical effect chance of Lesser Healing Wave, Healing Wave, and Chain Heal by 20%. Every time one of these spells crit, the stack is reduced by one. This means that after one crit the effect is reduced to +40%, after two it is reduced to +20%, and after three crits the buff is removed.
Since Chain Heal has a separate chance to crit on each target, it will remove one charge from the buff for each target it crits on. If the spell crits on all three targets, it will immediately consume all three charges of the Tidal Force buff.
It can be used to trigger the Ancestral Healing talent on a tank that is taking heavy damage. It will also trigger the Ancestral Awakening talent to provide even more healing in an emergency situation.