I play bluegrass guitar and 5 string bluegrass banjo ( Earl Scruggs Style) The rhythm guitar is playing what we call a "boom chuck". There is also a "boom chuck ah", as well as a "boom ah chuck ah". These are terms used to describe the movements. For example: Playing a G chord on the guitar; Boom is where you pick a down stroke on the 6th string, the chuck would be where you strum down usually catching the bottom 3 strings. Boom chuck a would be to pick a down stroke on the 6th string, then a down strum followed by an up strum. These rhythms are used to keep time. Most guitar players, whether they realize it or not, are counting in their heads. 1 2 3 4 or 1 & 2 & 3 & 4 & for 4/4 time . 1 2 or 1 & 2 & for 2/2 time. Most consider 1 2 3 4 to be down strokes and up strokes for & . Boom would be a down stroke, chuck would be a down stroke and the "ah' would be an upstroke. In my opinion, if I were playing an uke with bluegrass I would use down strums and up strums using all 4 strings. Boom and chuck would be a down strum using all 4 strings and the "ah" would be an up strum using all 4 strings. Put more strum emphasis on your boom down strum and less on the up strum. Hope this helps.
Tenors: Lanikai NKT, Honu KXXXe-inf, Wixom #142, Pono RTSH PCC cedar/rosewood
Baritones: Kala ASAC-B, Pono MB