A Quick Word On Technique

Aikido's technique is summed up by Ueshiba (In his later years) as-

"In aikido, there is absolutely no attack. To attack means that the spirit has already lost. We adhere to the principle of absolute nonresistance, that is to say, we do not oppose the attacker. Thus, there is no opponent in Aikido."

In practice this means the defender does not perform direct blocks (such as in Karate) or does not perform a counter strike against the attacker. Instead the practitioner will maneuver themselves to gain control of the opponents center of mass, which is commonly referred to as simply "center".  This control is accomplished by either applying force to the attacker's weaker joints (wrists, neck, elbows, etc) causing the attacker to lose their balance. Because of this the Tokyo Riot Police use Aikido as their martial art of choice because it is excellent for restraining an individual without causing permanent damage.

Morihei Ueshiba in his later years, which are regarded as where his most effective techniques emerged, began to put most of his energy into movement instead of blocking or using excessive force. This is why in many photos the practitioner does not look strained; the practitioner simply does not expend very much energy as there are no strikes or blocks. It requires very little force to be applied to something like a wrist to restrain someone.
(Ueshiba taking the balance of his partner)
Photo Credits: http://aikidojournal.com/wp-content/uploads/2016/01/morihei-ueshiba-old-aikikai.jpg