It's not a complete sentence, as a [verb]ing form cannot function on its own as a verb. I looked for examples, and the general meaning is that you're going to talk about something different for a moment. "Switching gears for a second, how are you preparing your company for the year 2000?" "Switching gears for a second: I'd like to reach back to the turn of the century..." "Switching gears for a second, I'd like to introduce you to Jack." (Google search) We can't use "switch gears" because that's a command. The [verb]ing form isn't a continuous form or an action at this moment because we need a form of "to be" if we want that meaning: "We are switching gears for a second". It isn't a gerund form either because it doesn't function as a noun here. Your phrase "Practicing English on day 2" uses a gerund, and this is how you'd present an activity as the title of a text. As for "switching gears for a second", you have a "participle phrase". That means the whole phrase provides extra information, but cannot function as a sentence alone. If you are at B2 level in English, this is the point where you would start seeing participle phrases in your study. It's a very useful way of adding information to a sentence without confusing the listener.

The "switching" and "practicing" are happening as we speak. It's happening in the moment (present continuous). We need "ing" here.

It is the verb however its the gerund form. This is because its describing an action that is still in the process of happening. As for your second question, "Practicing" sounds more correct to me as well.