By Kyabje Lama Zopa Rinpoche in Kopan Monastery, Nepal

Lama Zopa Rinpoche explained the visualization when doing tonglen (taking and giving) meditation in this excerpt from the 26th Kopan Course.

Lama Zopa Rinpoche's Online Advice Book
A Magnet for Bad Situations •
How to Practice Tong-len •
Become Special by Practicing Tong-len •
Daily Practice of Taking and Giving •