Explain aggregation 4 stages with group and slice

explain1.js

{
	"stages" : [
		{
			"$cursor" : {
				"queryPlanner" : {
					"plannerVersion" : 1,
					"namespace" : "agg.scores",
					"indexFilterSet" : false,
					"parsedQuery" : {
						"game" : {
							"$in" : [
								"G0",
								"G100",
								"G500"
							]
						}
					},
					"queryHash" : "3BADF9B3",
					"planCacheKey" : "1268FF8C",
					"winningPlan" : {
						"stage" : "PROJECTION_COVERED",
						"transformBy" : {
							"game" : 1,
							"player" : 1,
							"score" : 1,
							"_id" : 0
						},
						"inputStage" : {
							"stage" : "IXSCAN",
							"keyPattern" : {
								"game" : 1,
								"score" : -1,
								"player" : 1
							},
							"indexName" : "game_1_score_-1_player_1",
							"isMultiKey" : false,
							"multiKeyPaths" : {
								"game" : [ ],
								"score" : [ ],
								"player" : [ ]
							},
							"isUnique" : false,
							"isSparse" : false,
							"isPartial" : false,
							"indexVersion" : 2,
							"direction" : "forward",
							"indexBounds" : {
								"game" : [
									"[\"G0\", \"G0\"]",
									"[\"G100\", \"G100\"]",
									"[\"G500\", \"G500\"]"
								],
								"score" : [
									"[MaxKey, MinKey]"
								],
								"player" : [
									"[MinKey, MaxKey]"
								]
							}
						}
					},
					"rejectedPlans" : [ ]
				},
				"executionStats" : {
					"executionSuccess" : true,
					"nReturned" : 78051,
					"executionTimeMillis" : 185,
					"totalKeysExamined" : 78053,
					"totalDocsExamined" : 0,
					"executionStages" : {
						"stage" : "PROJECTION_COVERED",
						"nReturned" : 78051,
						"executionTimeMillisEstimate" : 9,
						"works" : 78053,
						"advanced" : 78051,
						"needTime" : 1,
						"needYield" : 0,
						"saveState" : 81,
						"restoreState" : 81,
						"isEOF" : 1,
						"transformBy" : {
							"game" : 1,
							"player" : 1,
							"score" : 1,
							"_id" : 0
						},
						"inputStage" : {
							"stage" : "IXSCAN",
							"nReturned" : 78051,
							"executionTimeMillisEstimate" : 4,
							"works" : 78053,
							"advanced" : 78051,
							"needTime" : 1,
							"needYield" : 0,
							"saveState" : 81,
							"restoreState" : 81,
							"isEOF" : 1,
							"keyPattern" : {
								"game" : 1,
								"score" : -1,
								"player" : 1
							},
							"indexName" : "game_1_score_-1_player_1",
							"isMultiKey" : false,
							"multiKeyPaths" : {
								"game" : [ ],
								"score" : [ ],
								"player" : [ ]
							},
							"isUnique" : false,
							"isSparse" : false,
							"isPartial" : false,
							"indexVersion" : 2,
							"direction" : "forward",
							"indexBounds" : {
								"game" : [
									"[\"G0\", \"G0\"]",
									"[\"G100\", \"G100\"]",
									"[\"G500\", \"G500\"]"
								],
								"score" : [
									"[MaxKey, MinKey]"
								],
								"player" : [
									"[MinKey, MaxKey]"
								]
							},
							"keysExamined" : 78053,
							"seeks" : 2,
							"dupsTested" : 0,
							"dupsDropped" : 0
						}
					}
				}
			},
			"nReturned" : NumberLong(78051),
			"executionTimeMillisEstimate" : NumberLong(100)
		},
		{
			"$group" : {
				"_id" : "$game",
				"top5" : {
					"$push" : {
						"player" : "$player",
						"score" : "$score"
					}
				}
			},
			"nReturned" : NumberLong(3),
			"executionTimeMillisEstimate" : NumberLong(177)
		},
		{
			"$set" : {
				"top5" : {
					"$slice" : [
						"$top5",
						{
							"$const" : 0
						},
						{
							"$const" : 5
						}
					]
				}
			},
			"nReturned" : NumberLong(3),
			"executionTimeMillisEstimate" : NumberLong(177)
		},
		{
			"$sort" : {
				"sortKey" : {
					"_id" : 1
				}
			},
			"nReturned" : NumberLong(3),
			"executionTimeMillisEstimate" : NumberLong(177)
		}
	],
	"serverInfo" : {
		"host" : "asyas-mbp-4.lan",
		"port" : 27017,
		"version" : "4.4.0-rc7",
		"gitVersion" : "b79b53f55a5c148fd297b81a45c08d08e2cf8f94"
	},
	"ok" : 1
}