I know "jump off the deep end" means to abruptly step into something but I'd like to know what it means in terms of actual physical jumping – because you usually "jump off" something not deep. What does a native speaker imagine when hearing the phrase?
Learn English – Where does “jump off the deep end” come from
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Related Solutions
Capping something off means putting a cap on it, whether a metaphorical cap (like capping your career off by becoming the president of your company) or a literal, physical cap such as the cap you put on the end of a pipe.
There are many sorts of pipe cap, of many different materials. Some screw onto a threaded pipe, others are simply pushed on and held in place by friction or by clamps. What sort of cap you use will depend on what sort of pipe you're working with and how much pressure has to be resisted. Here's a typical threaded cap on a water pipe: 
When music was playing on a stereo or in the garage, she could tune it out or switch it off . . .
Switch off is used literally to designate turning or flipping a switch to end the functioning of a machine or appliance. But it may also be used metaphorically of mental function: we might speak of "switching off" your attention to some phenomenon.
In your example, where actual music is being played, switch off is probably meant literally, since it contrasts with tune out, a similar expression (drawn from 'tuning' a radio to a particular channel) which today is almost always employed metaphorically.
PARAPHRASE:
When music was playing on a stereo or in the garage, she could ignore it or turn the stereo off . . .
Best Answer
The relates to jumping (or diving) into the deep end of a swimming pool.
In the deep end of the pool, the water is too deep for you to stand on the bottom, so you are forced to swim. It's a metaphor for coping with the unfamiliar, especially when you are not properly prepared. "Deep water" is a metaphor for the unknown, so when you jump into deep water you don't know what might happen, or what might be lurking under the surface.
A related idiom is sink or swim. When you jump off the deep end, you have choice but to learn to swim, otherwise you're going to drown.
In addition, go (or jump) off the deep end has become synonymous with go crazy, or lose one's head:
The connection here to swimming pools is less clear, although it's probably connected with the idea of deep water being (literally) unfathomable, meaning that you have no idea what someone who has "gone off the deep end" is going to do.