I think that he means the quasi trill pattern when, let's say, you play F-sharp and G over and over again with a staccato touch, F-sharp with LH and G with RH. Like a staccato trill.You have it in Rach2 2nd movement, or Liszt HR2 for example.
so just slow practicing?i feel like one side of my brain is less supportive than the other side when i play this