The answer is: he may do so. Yet if he is able to endure the itch, he should remain patient. But if it distracts his heart, he should scratch it, removing what prevents him from humility and attentiveness. It is well known that when a person scratches an itch, it cools and subsides.
“If he prolongs an action, as customarily understood, without necessity and without interruption, the prayer is invalidated, even if done forgetfully …”
His statement, “If he prolongs an action, as customarily understood”: “as customarily understood” is in the accusative by omission of the preposition; that is, an action considered prolonged according to customary practice.
Customary practice means what people are accustomed to and familiar with.
His statement, “without necessity,” means without being compelled to prolong the action. For example, if a predatory beast attacks him and will eat him unless he deals with it and fends it off; or if a snake attacks him and will bite him unless he repels it; or likewise a scorpion. Such action is necessary, and the prayer is not invalidated by it.
His statement, “and without interruption,” means that the action is not scattered about, but is continuous—for example, within a single unit of prayer. This differs from a person who makes one movement in the first unit, another in the second, another in the third, and another in the fourth. Their total may be many, but each individual movement is slight; in this case, the prayer is not invalidated. However, if the movements are continuous and numerous, they invalidate the prayer.
Thus, there are three conditions for the prayer to be invalidated by movement:
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That it be customarily considered prolonged.
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That it not be due to necessity.
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That it be continuous, without interruption.
If these three conditions are found in an action, it invalidates the prayer, because it is movement unrelated to the prayer and is incompatible with it, like speech. Whatever is incompatible with the prayer invalidates it.