ID stairhop is a tactic whereby the player repeatedly ascends and descends a particular set of levels in the Infinite Dungeon for the purposes of collecting loot or, occasionally, finding a particular monster. This method is maximally efficient for collecting loot generated from floor drops: for each turn of stairhopping, the PC reveals ~20-30 squares of new floorspace to hunt for items. This method is also relatively low-risk, since only monsters generated directly adjacent to the stairs are capable of engaging the PC (unless he wanders off the stairs to pick up an item).
As with a number of other 'scumming' tactics employed by players of ADOM version 1.1.1 (or earlier), ID stairhopping was 'fixed' in 1.2.0; now items can no longer be generated in stair rooms in the Infinite Dungeon. It remains a valid tactic for prerelease 6 and below.
There are several floors in the ID that are normally employed for stairhopping, depending on what the PC is wishing to find. Stairhopping between I1 and I2 will produce danger level 1 items, which include some useful low level armor, rings, waterproof blankets, and, most importantly, all spellbooks. Particularly for natural casters such as Wizards and Priests, the number of floor dropped spellbooks is considerable, and they may accumulate large numbers of spells quickly by stairhopping.
Another useful area to stairhop is between I3 and I4, as these floors will produce blink dogs. Blink dog corpses provide the extremely useful intrinsic teleport control, which is particularly important to casters, and speed runners, but generally worthwhile for any PC to obtain.
Finally, at substantially deeper depths, players often search I24-I25 for the prized ring of djinni summoning in hopes of obtaining a wish. It should be noted that this is a considerably more dangerous option, since the PC must successfully descend numerous ID levels to reach this point, meaning that the monsters they face will be considerably stronger. Furthermore, I24-25 are corrupting levels, so the PC will accrue some corruption while pursuing this.
In principle, any level of the ID can be used to generate loot. The only limitation on this scumming tactic is really the patience of the player. Low level players should avoid stairhopping as Strained! or even Strained, as this makes you susceptible to falling down the stairs and breaking your neck.