text: Table 4 indicates that the best results are obtained by 
  sampling x using a multi-block strategy, wherein the 
  network is forced to make predictions after removing 
  large continuous blocks in the video. When x is only 
  sampled from the first few frames of the video, as in 
  the causal multi-block strategy, we observe a decrease 
  in downstream performances. Finally, the random-tube 
  strategy, wherein 90% of the tubes in the video are ran- 
  domly masked, leads to features of low-semantic quality 
  when combined with our feature prediction objective. 