February 2016 (perspective of an assistant professor)
A forcing function is an event that nudges you to take some immediate action that perhaps you wouldn't have taken otherwise. Setting up the proper forcing functions have been vital for my own productivity over the years, since they provide motivation via concrete deadlines.
For instance, scheduling an invited research talk is a forcing function for me to polish the way that I present a particular project. After all, I don't want to embarrass myself by giving a bad talk in front of people I respect. Invited talks are also a great forcing function for me to read up on the research of the people who invited me, since I want to have engaging conversations about their work as well. I'm normally a bit lazy about reading research papers, so I've found that not wanting to embarrass myself when talking to colleagues is a great forcing function to read up on their work and think about how it relates to mine.
The best part about well-designed forcing functions is that they result in long-term benefits that transcend the event itself. In the invited talk example, I don't just spend time improving my talk and reading people's papers to optimize for that specific event; rather, the knowledge that I gain makes me into a more experienced and knowledgeable researcher overall.