Hi,

I saw some of the runs and I saw you guy using spike glitch (in 2.2 and 3.1 or something like that)

So you can walk on the spikes and not getting hit at all for the whole level.

How does it work ?

It's actually really simple to perform, you just jump and land on the very edge pixel of a spikes hitbox. Unfortunately it's a frame perfect trick, so not the easiest thing to pull off everywhere you'd like in a run.
As for the reason why this works I'm not really sure!