The phrase "jumped in with feet" often refers to a situation where someone eagerly or recklessly gets involved without fully considering the consequences. Some synonyms for this phrase could include "dived in headfirst," "plunged in," "rushed in," "hastily joined in," "charged in," "pounced on," or "leaped at." These synonyms emphasize the impulsive and enthusiastic nature of the action, but may also suggest a lack of caution or forethought. Depending on the context, using these synonyms might add depth or nuance to your writing, conveying a particular tone or attitude towards the action.