Happy Sunday Sister! :)
Hope you don't mind me replying.
The answer is somewhat simple, though it's a phrase that's been used by some that believe that it's some 'altered state of consciousness' that actually would be categorized as a counterfeit experience.
We know that on Patmos, John says that he was "in the spirit on The LORD's day" and he says that as if it were something that was not an unusual thing for him while he prayed. What happened while he was in the spirit was unusual, of course and he was able to write it all out for us in his Revelation.
When we're in prayer, we want it to be "in spirit" and we normally start off with our own words but as we are praying our own words, our focus becomes - or should become, completely on Him and Up to Where He Is.
At some point, we become more in line with what His Spirit would be praying and we understand that the Spirit is now praying through us ... whether it be in our native tongue or not doesn't quite matter - it's the fact that we know that we're being aided by His Spirit and that we haven't premeditated the words that are coming from us.
When we're praying in the Spirit - we'll experience the appropriate emotions of what the Spirit is wanting us to feel. It can be joy, an awesome reverence for The Almighty, a burden for souls or a certain person or any other burden the LORD through His Spirit lays on us. We can begin to cry or just grieve and that can feel as if something is being released. A burden that the Holy Spirit wanted to intercede about 'through' us.
It can be Praise that ushers forth or ends in Praise, but whatever the Spirit wants to do through our time of prayer - He'll use words, groans, thoughts, feelings and emotions to express through us.
He may even give mind pictures/visuals or other means to show us what or where His Mind is, that He wants to show us - but it's not something that we have to have in order to be a channel in which the Holy Spirit can use for supplications, intercession, praise and even healing to ourselves.
People will give their own experiences and that can be good or bad -- only in that our GOD works differently with each of His Children and not all are required to have visions, etc.. The main point is, is that the person knows that though they are consciously permitting the Holy Spirit to do what He chooses through our feelings, mouths, emotions, etc... 'we' haven't lost control like some occultists do.
We can have visuals but they are not to be put as a requirement or made a definition of being 'caught up in the Spirit'.
Bragging of what one has experienced while in 'HIS' Spirit throws a up red flag up to me, that the person is starting to get in the flesh, but a person can, if they feel called by Him to share something that they feel that The LORD has something to say to The Church from their time in prayer, when they knew it was His Spirit directing it.
Any 'words' or visuals that are shared with another needs to be shared 'only' because The LORD is giving direction to share these things and not for a "WOW" directing toward ourselves.
The main thing about being 'caught up in The Spirit' is that it always has the affect of leaving the person with that Holy Awe and Reverence for Who He Is ... and that's can be an indicator that it is the true experience.
The HOLINESS of GOD would be the number 1 thing that the person comes away with, when truly in or experiencing His Presence - which is in other words, what being in His Spirit would produce. We're in His Presence though not exactly like John was --- because John went from praying in the Spirit, to being caught up further than we are, 'after' he heard a Voice behind him and we know the rest from what he recorded.
We could have a mini-experience of what John saw, heard, etc, but be careful if anyone compares themselves to John or all that he wrote :)
Bottomline - it's ALL for HIS Glory and not the person's.
GOD Bless!