Problems with the abdominal area while practicing Vipassana

I have been practising Vipassana for 5 months since going on a 10 day course. In answering someone's question on increased salivation during a 10 day Vipassana course, the respondent stated that the abdomen was the anchor that stopped us from flying away. I was very interested in that because when I meditate I sometimes feel there is a huge stone in my stomach region that won't let me go. Occasionally if I struggle really hard I feel I can break the link to that weight and then I seem to pop out into a huge very blissful area. This occurs only rarely as it is very difficult to do. To do it, I feel I have had to give up on everything and 'throw myself off a cliff' I feel there is a lot of fear in this stomach area and a tightness like a fist. I always find it very difficult to feel sensations in this area when practicing the Vipassana. Can anyone help explain further about the abdomen being an anchor to stopping one from flying away and perhaps answer it in relation to the experiences I mention above. Thank you for your time.