Forward Roll is the main ingredient that makes Bluegrass banjo sound different from all other playing styles. Many Old-Time players used forward roll sparingly but Bluegrass players use it all the time.
Forward Roll is only three notes played round and round in a loop. Starting with the Thumb, the next note is picked with the Index finger, the next with the Middle finger and then repeat.
Starting with the Index the next two notes would be Middle followed by Thumb.
Starting with the Middle finger the next two notes would be picked by the Thumb then the Index.
Because most music is in four four time, three notes into four four doesn’t go so Forward Roll crosses over the rhythm all the time, giving the characteristic Bluegrass sound.