TL;DR Yes. For some of it.
I was raised in a Church of God in Christ family (even though we didn't spend a lot of time in actual C.O.G.I.C. churches because ... reasons).
It's very easy for someone unfamiliar to walk into a Pentecostal service and misunderstand what they're seeing. It's also kind of hard to describe some of these things without sounding a little silly. Pentecostal services are very emotionally charged. It is a part of the practice of worship to let your conscious self fade away in order to allow the Holy Spirit to take control. This is based in part on James 4:7-10.
Submit yourselves, then, to God. Resist the devil, and he will flee from you. Come near to God and he will come near to you. Wash your hands, you sinners, and purify your hearts, you double-minded. Grieve, mourn and wail. Change your laughter to mourning and your joy to gloom. Humble yourselves before the Lord, and he will lift you up.
James 4:7-10 (NIV)
While there's no place in the Bible that specifically says "And thou shalt run around the church, jump the pews, and stuff" there are plenty of passages that describe singing and shouting praises to God. Palm Sunday is a perfect example. Another passage that is often quoted is the one about David dancing before the Lord.
If you've ever gotten yourself so worked up emotionally that you just had to move then you have a sense of what's happening to people when they are filled with the Holy Spirit. From my own experience it's a lot like being drunk. Your inhibitions fade away. You are overcome with emotion and then that emotion just has to get out. And so you dance, cry, shout, sing, run or whatever. On the day of Pentecost people were so overcome by the Holy Spirit that they were accused of being drunk at 9:00 AM (Acts 2).
Now, the only times I've ever seen or heard of anyone having a seizure or convulsions during service were either:
- someone was faking it and trying to hard (it happens ... kind of a lot)
- someone was having a legitimate medical seizure (this one I saw and the elders recognized it immediately and called 911)
- (this one is really really rare) someone was so tied up in something so horrible emotionally that the encounter with the Holy Spirit manifested a powerful physical reaction. Given my experience with modern mental health, I'd say it was that a person with a serious emotional disorder experiencing a powerful physical reaction to their condition combined with the emotional experience of worship. Old folks would say it was the Holy Spirit fighting with demons for a persons soul which if you think about it is pretty accurate. And that literally happened in scripture at least twice that I can think of.
Pentecostals are great jokers and are easy to make fun of. And combined with what they actually do it can be hard to draw the line between what really happened and what was a joke. I advise you to specifically ask your friends about what they've actually seen or better yet, go to church with one of them.