Every drum has something called an air vent. This is a small hole on the shell of a drum that allows air to escape, which allows the drum to resonate and sustain. By placing a tube (I use plastic in the video) into the air vent, one can blow air into the drum, causing the pitch to rise, and subsequently fall when the air slows down or stops. This is generally a neat effect, and also can be a great substitute for timpani. That’s usually not the case, but for a piece that asks for timpani solely for the glissandi effect, an air-bent drum an be a much cheaper solution.
I don’t speak for everyone, but most percussionists aren’t particularly practiced in the art of breath control. For best results, I’d suggest using this technique sparingly, and asking for little in terms of interval shifts and duration. Hitting specific pitches is a challenge; it requires a good deal of breath control, and is also at the mercy of each drum’s finite range.
The notation of this technique is an enigma for me, as there are few examples and many potential issues. One very basic suggestion I can give is below, representing a general gliss up and down the drum’s range.
Going for an exact pitch, of course, can create additional problems. It might become difficult to notate in the event that you want a specific pitch within an unpitched setup. Additionally, it further restricts the drums that may be used, as each drum will have its own unique range. I don’t feel good about offering a catch-all solution, so I would just say to try and use this technique in a way that allows it to be conveyed as simply as possible.