suck in one's cheeks

[Hongdung]

I don't understand the act "suck in one's cheeks". Could you help me to explain this phrase or find out its synonym? Thank you very much.
Jung

[Kitosdad]

An action performed by someone who been alarmed or startled.

An influx of breath, followed by a surprised exclamation.
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Keep it simple ... Keep it interesting.

[Cerberus™]

"To suck" can be the act of inhaling, causing air or liquid to enter your mouth. Inhaling is expanding your lungs, which causes low pressure in your respiratory system as opposed to the air outside of your body. This low pressure in your mouth may make the outside air push harder on your cheeks, causing them to go "in", that is, become hollow as viewed from the outside. Low pressure in the mouth can also be brought about locally. Sucking in could be considered the opposite of blowing out.

[Jamie (K)]

He is sucking the air out of his mouth and this makes his cheeks look skinny.