Buffer.cs

using System;
using System.Collections.Generic;
using System.Linq;

static class Extentions
{
  public static IEnumerable<IEnumerable<T>> Buffer<T>(this IEnumerable<T> source, Func<T, bool> func)
  {
    while (source.Any())
    {
      yield return source.TakeWhile(x => !func(x));
      source = source.SkipWhile(x => !func(x)).Skip(1);
    }
  }
}

class Program
{
  static void Main(string[] args)
  {
    var nums = new[] { 1, 2, -1, 5, 6, 8, -1, 11, 2, 4, 6, -1, 9 };

    nums.Buffer(x => x == -1).ForEach(x => Console.WriteLine(string.Join(",\t", x)));
  }
}

output:

1,      2
5,      6,      8
11,     2,      4,      6
9