“Informal, or formative assessments are about checking for understanding in an effective way in order to guide instruction. They are used during instruction rather than at the end of a unit or course of study. And if we use them correctly, and often, yes, there is a chance instruction will slow when we discover we need to re-teach or review material the students wholly “did not get” — and that’s okay. Because sometimes we have to slow down in order to go quickly”
Here is another short, but informative article on the power of formative assessment. The author also includes a few formative tools she has used with her students.