Wiktionary
jump in
vb. 1 To enter something quickly. Usually a mode of transport. 2 (context slang English) To initiate into a club, usually a gang, with violence.
Usage examples of "jump in".
Another agent may then jump in at the right page to effect a rescue.
The last two times he had done this, the basket was bobbing and weaving so much, it took several flybys before he actually had time to jump in.
Morvrin's snort was loud enough to make everyone jump in their chairs.
We have to keep an eye on everything, and be ready to jump in any direction no matter which item on the agenda comes up first.