Yíxià(r) 一下(儿)/一下(兒) is a verbal measure word used after a verb to describe an action. It modifies the verb and describes the action as being brief or tentative.
Note, the ‘儿´ after yíxià(r) 一下(儿)/一下(兒) is optional.

Structure
Basic form
Yíxià(r) 一下(儿)/一下(兒) is placed after the verb.
If there is an object in the sentence, the object is placed after yíxià(r) 一下(儿)/一下(兒).
When the object is a personal pronoun, such as wǒ 我 ‘I; me’, the object is usually placed between the verb and yíxià(r) 一下(儿)/一下(兒).
The verb describing the action can be preceded by modal verbs such as xiǎng 想 ‘want‘, yào 要 ‘want; need‘, or yīnggāi 应该/應該 ‘should; ought‘ to express different ways of refusal, reminders, or commands. The modal verb is placed before the main verb and yíxià(r) 一下(儿)/一下(兒) is still placed after the main verb.
Question
There are two ways to form a yes-no question.
One way is to add the question particle ma 吗/嗎 at the end of the sentence.
If the sentence contains a modal verb, a yes-no question can be formed by combining the affirmative and negative forms of the modal verb. Usually, a modal verb is used in this type of question to soften the tone.
With adverbs
Adverbs, such as dōu 都 ‘both, all’ and yě 也 ‘also’ are placed before the verb.
If there is a modal verb in the sentence, adverbs usually precede the modal verb.
With prepositional phrase
A prepositional phrase should be placed before the main verb and after the modal verb if is there is any.
Functions
1. Yíxià(r) 一下(儿)/一下(兒) indicating a brief action
2. Yíxià(r) 一下(儿)/一下(兒) softening the tone
Yíxià(r) 一下(儿)/一下(兒) softens the tone of requests or commands, making them sound less direct and more polite. The subject is soften left out in imperative sentences or when it is clear in the context.
3. Yíxià(r) 一下(儿)/一下(兒) expressing ‘to give something a try’
Yíxià(r) 一下(儿)/一下(兒) suggests performing an action as a quick attempt or experiment, often to see how it goes. In this usage, yíxià(r) 一下(儿)/一下(兒) functions similarly to verb reduplication, making the action seem lighter or more casual.
Also check: Reduplication of verbs
Also check: Yìdiǎn(r) 一点(儿) v.s. yíxià(r) 一下(儿)
Also check: Yìdiǎn(r) 一点(儿)