Rabbi Simchah Bunim of Pshische related the following example of how simchah empowers a person with new strength.
Once while in the city of Dantzig, he saw a man drowning.There was no way Reb Simcha or anyone could save him; the person had to summon up the inner energy to fight the waves.
At first, the man was struggling vigorously. But the challenge was great. His strength began to ebb, and Reb Simcha saw a look of despair begin to form on the man's face; he was prepared to give up. With a slight smile, Reb Simcha shouted to the man: "give regards to the Livyoson"
A light smile broke out on the drowning man's face. With fresh vitality, he renewed his struggle and persevered until he was able to save himself. The smile had brought on a surge of new energy that saved his life