Explain the mechanism responsible for summation of contractions and the increase in height of contraction when the stimulus frequency is increased.

The higher the stimulation frequency, the contractions will be more continuous and even additive.

Continuous stimuli reactivate muscle contraction once the action potential cycle is complete.

Explanation:

Muscle contractions can be continuous or cumulative once the action potential curve drops, since the action potential at the top of the mountain is in a refractory period.

I knit a graph of what cumulative contractions are like due to continuous stimulation.

In the graph I leave you pointed out some references:

mr = refractory moment

mnr = non-refractory moment (where the new stimulation is accepted and a more powerful cumulative muscle contraction is generated)

PA = potential for muscle action or contraction itself.

meteorologists would compare a new weather map with one 24 h old to see how fast a front is moving.