一下子 could also mean with just one try, in one go 他一下子就把瓶盖打开了 He opened the lid of bottle with just one try. 他一下子就讲明白了。 He explained this hard point clearly very soon. 一下子 means you successfully do something without much effort , or very fast, or very easily, while most other people need to try very hard but they are still hardly able to work it out.

There are two major meanings of "一下子“ we use a lot： 1. to make something successful easily, without using too much effort; or to achieve success easily at one stroke. eg1. 他一下子就记住了整段乐谱。/ 他 不能一下子 记住整段乐谱。 eg2. 那场演出让他一下子出名了。/ 那场演出 不能让他一下子 出名。 2. suddenly , immediately, or something happens in short period of time. eg1. 一队鸭子突然出现在马路上，汽车一下子都停了下来。 eg2. 他一下子清醒了。 eg3. 花一下子都开了。 eg4. 人一下子就老了。

"一下子 (一下)" has multiple meanings "子" can be omitted. "一下子"/"一下"， either is fine 1. in one go/all at once e.g. She got the answer to the riddle in one go 她一下(子)就猜中了（谜语的）谜底 this meaning of "一下子" emphasizes on doing something quickly, effortlessly, and successfully 2. in a short while/for a little bit (time wise) e.g. Wait up! 等一下（子）！ e.g. She paused (for a bit) before going back to work 她停顿了一下(子），才继续工作。 3. all of a sudden e.g. She suddenly sat up. 她一下(子)坐了起来

一下子 = 马上