What does it feel like to need a HUGE poop?

i've never really pooped a big, soft, solid log b4 and have forgotten what it feels like to really "need" to go?! Can anybody tell me what it feels like and how i can do this?? i am 20 and have quite an unhealthy diet... i need to go now, but it is highly likely that it will be pebbles... i'm thinking about holding it in-- so it will eventually build up into a big turd... Sorry if i'm grossing you out.. its just a question to do with health..