The bridge of merit can occur by three factors:
Pure faith
Highly virtuous deed
Pure field of merit
Before teaching Vipassana or even after starting the practice, I've never experienced the bridge of merit. One reason is because my merit making was mostly not pure as I hoped that good result would come to me quickly. Although I got a chance to make merit by giving directly to Luang Ta Maha Bua with faith, deep down I expected the merit to bring me success. This might be because I never had spiritual teacher or never met with true friends to guide me the right way. Therefore, I never knew about pure giving.
Until I became a serious Vipassana meditator then I had taught myself everything and teach my students to know how to be a true giver, not asking for anything but to give and give with gratitude. I teach them to make a wish at appropriate time since they have duties and they need support for their life to move on.
The incident of the bridge of merit happened for the first time on the day we went to worship the little Buddha's footprint which had been abandoned for a long time. So, we, from Techo Vipassana, decided to restore the place in order to pay utmost respect to Buddha and bring faith back to the local people. On that day, when the students who participated in the ceremony listened to my prayer, they had strong faith and thought of the Buddha. This caused a powerful stream of the bridge of merit, allowing them to join the merit making which would be great benefits for their life.
What never happened had happened. What should happen only in peaceful places like Dhamma retreat or temple, had happened in an unexpected place. Dhamma always showed us the miracle. Dhamma truly occurs at the heart and wherever that person is, he or she can make the place sacred.
Another thing is that being among true friends would bring the stream of merit. Associating with true friends surely brings us great benefits.
Doing things without making sacrifice is not virtuous deed.
Doing things with expectation for return is investing not making merit.
Wrong investment brings loss, unwise investment is not worth it.
If you haven’t got into the meaning of the word "sacrifice", please reconsider your path. Have you truly sacrificed anything or you just hold on to everything? If you've never sacrificed, please do so. The true giver will always receive. This is the rule of Dhamma, the rule of the one who overcomes impurities in the mind.
May you see the path and choose to walk the path to see Dhamma.
Source: An extract from Master's Teaching "Dhamma That Is Never Heard" (13 November 2013)