Reflexes are not all sudden and catastrophic in effect. Stroking the sole of a foot with a blunt object normally causes an involuntary flex downward of the toes, for instance (if reversed, it is the Babinski sign -- and may mean CNS damage).

I am aware that aiki-age it is also done slowly and do so with greater or lesser success. The reflexes we are talking have two main types and operate in two different ways. Stretch reflexes and tendon reflexes, and which can trigger and inhibit muscle actions -- first, by prompting contraction on the agonist or synergist muscles, and second, by inhibiting the antagonist muscles on the opposite side of the limb (reciprocal inhibition).

The knee-tap reflex test is the kind of sharp stretch reflex resulting in sharp reaction -- not unlike what the panel sequence showed. But lower magnitude forces triggering the muscles spindles create more subtle reflex effects. The Jendrassick effect shows there are known correlated stretch reflex connections between the upper and lower limbs -- nikkyo and sankyo exploit this in precisely inverse ways -- mainly because the twist in each is reverse from the other -- and so differ in which muscle spindles are on the synergist versus the antagonist side of the spiral line of stretch (shear tension) -- that triggers the reflexive action. The reflexes also have some left-right connections as well, which are typically inverse (contralateral) , i.e. - flex on one side vs. extend on the other side (an aspect of ten-chi, FWIW).

When aiki-age is done slowly the predominating effect is less from the triggering of, say, the extensor muscles to actively lift you, but an inhibition of the antagonist flexors, resulting in an imbalance of forces on the limb such that the unbalanced normal tone of the extensors now lifts you -- mainly because it is unopposed by the balancing force on the opposite side of the limb. That's why it seems hard to notice why it is happening or control it, because the reflex action is not doing anything to lift you -- but merely stopping part of you from doing what it normally does without you noticing to begin with.