It can be easy to forget about how high the stack is getting. Left unchecked, overstacking can lead to a topout prematurely. To "skim" means to clear Singles, Doubles, or Tetrises over the Tetrising column. This gracefully lowers the surface's height without interfering with the Tetrising column. It is also useful for handling awkward pieces that may not fit anywhere.
Higher stacks makes you more vulnerable to topping out due to surface instabilities. It gives you less time and mobility. At that point, even minor threats can become deadly. That is why it is important to get a feel for how high you are willing to go. Once you reach that point, begin preparing in case the I-piece does not come in time.
Leave an empty nook alongside the edge. This provides a safety net against overstacking.
Even a shallow notch enables skimming opportunities.
Here are some examples.
It is usually better to maintain a notch even after you skim. Avoid raising the surface next to the Tetris column when possible.
It is not always necessary to clear lines right away with the piece that covers the Tetrising column. It can serve as a temporary placeholder when needed. This is useful for when you get the piece you know will work, but too early. Placing it anywhere else could mean not being able to skim should you not get another one soon. However, there is a trade-off between temporarily covering the Tetrising column and ensuring skimming ability. You must weigh the risk of not being able to Tetris (should an I-piece come) against the risk of not having placed a fail-safe skimming piece when you had the chance.
Awkward pieces can be dealt with this way as well.
Lastly, skimming allows you to surgically fix holes near the surface without needing to destroy the entire stack.