Never grew fastberry but applied hst on other fast buds autos with great result.you can't go wrong with hst unless you completely break the stem.even if you break,you can tape it and itll probably recover.hst makes stems bigger,stronger,ends with greater yields.my fav training so far.just break the inner muscles and you are safe
By the way,you can hst your plant even after 3-4 weeks into flowering with no problem.bent stems and tops will position themselves to the light in a night