Diego Velasquez's portrait of Juan de Pareja, 1650
The method is ably explained and demonstrated in Free Art Lesson – James Sulkowski – Underpainting, a five-minute YouTube video.
Notice first how Sulkowski holds his brush – far enough back – and paints using his whole arm, not just his hand and wrist.
"The reason for this technique," he explains, speaking of the grisaille, "is to get more depth and luminosity." He also demonstrates, briefly, blending edges with a fan brush (yes, fan brushes are meant for blending, not for painting generic "foliage"). This is a wonderful demonstration of painting light and shadow on a form using darks and lights. Although he is using oil paints rather than acrylics, you can get a longer drying time with acrylics by either using a retarding or slow-drying medium, or by using Golden's Open Acrylics, which give you three to four days of blending time before the paint dries.
He continues demonstrating how glazes are used over the grisaille in Taking the Mystery Out of Glazing, a second five-minute video, showing how classical painters like Velasquez used this process to create paintings like the portrait of Juan de Pareja. He also demonstrates using a mahl stick on the painting's finest detail. If you are interested in classical realism, you definitely want to watch these two videos – they will definitely help you in your painting process.
You can also see James Sulkowski's work at his website www.jamessulkowski.com.