I'm gonna go through this step by step. It's a complex process.
Find a bar which is on the left wall. So on the left side of the block it is on. The glitch doesn't work if it's a different direction. Grab onto the bar. This is the tricky part. Hold the right analog stick right. At the same time, hold down left on the D-Pad.
If done correctly, yopur guy should spazz out and be all weird for as long as you are holding right on the right analog stick and left on the D-Pad whilst on the bar. During this seizure you are invincible. Cannot be punched, cannot be killed by any weapon. The only way to get out of this is to release the analog stick or D-Pad.
And there you have it.
Just a side note, whilst having a spasm you can move the right analog stick up or down slightly to move up or down the bar respectively.
Not anything major, just thought I should share this with you guys.