The Rambam in Hilchos Chometz U'Matza (1:3) says that if one would acquire chometz or make chometz on pesach they would receive malkus since it is considered an active violation of בל יראה ובל ימצא. When learning this Rambam over Pesach, I was troubled by the assumption that one can actively make chometz. It seems that chimutz is a chemical process that occurs by itself, how can one violate the making of chometz by doing an action?
R. Yakov Emded (sheilas ya'avetz, 1:16 - end of teshuva) raises this question on the Rambam. Why would this be considered doing an action? Even if one were to add yeast, it doesn't turn into chometz immediately upon the adding of yeast! This should be similar to a situation of סוף חמה לבא which is considered a grama, not a direct action? He suggests that perhaps the Rambam is speaking of one who causes the chimutz to occur through the heat of his hands, which would be considered an action.
In light of the gemara today, it seems that the Rambam could be understood. The gemara says that by placing yeast on dough and waiting until the chimutz occurs by itself, one would be chayev similar to roasting meat on shabbos where the cooking occurs by itself. The implication of the gemara is that one would even be chayev malkus. The Rambam (hilchos ma'aseh korbanos 12:16) writes exactly that - הניח שאור ע"ג העיסה והלך וישב לו ונתחמצה מאליה לוקה שהנחת השאור הוא המעשה
The Rambam understands that the chiddush of Rav Ami is not merely that one is in violation of baking the korban mincha as chometz, but that it is considered an action worthy of malkus similar to the shabbos violations. The act for which you are chayev is the placing of the yeast on the dough which will inevitably cause the dough to rise. This is considered an action worthy of malkus. Therefore, the Rambam in chometz u'matza who talks about malkus for creating chometz on pesach would also be speaking about placing yeast on the dough. But, if one were to just mix flour and water and wait until it rises, the gemara seems to hold that it wouldn't be considered a significant enough of an action to be chayev malkus for לא תאפה חמץ or בל יראה.